The blastostyles, gonophores and gonothecae furnish a series of variations which can best be considered as so many stages of evolution.Stage 1, seen inObelia. Numerous medusae are budded successively within the gonotheca and set free; they swim off and mature in the open sea (Allman [1], p. 48, figs. 18, 19).Stage 2, seen inGonothyraea. Medusae, so-called “meconidia,” are budded but not liberated; each in turn, when it reaches sexual maturity, is protruded from the gonotheca by elongation of the stalk, and sets free the embryos, after which it withers and is replaced by another (Allman [1], p. 57, fig. 28).Stage 3, seen inSertularia.—The gonophores are reduced in varying degree, it may be to sporosacs; they are budded successively from the blastostyle, and each in turn, when ripe, protrudes the spadix through the gonotheca (fig. 57, A, B). The spadix forms a gelatinous cyst, the so-called acrocyst (ac), external to the gonotheca (gth), enclosing and protecting the embryos. Then the spadix withers, leaving the embryos in the acrocyst, which may be further protected by a so-called marsupium, a structure formed by tentacle-like processes growing out from the blastostyle to enclose the acrocyst, each such process being covered by perisarc like a glove-finger secreted by it (fig. 57, C). (Allman [1], pp. 50, 51, figs. 21-24; Weismann [58], p. 170, pl. ix., figs. 7, 8.)Stage 4, seen inPlumularidae.—The generative elements are produced in structures termed corbulae, formed by reduction and modification of branches of the colony. Each corbula contains a central row of blastostyles enclosed and protected by lateral rows of branches representing stunted buds (Allman [1], p. 66, fig. 30).
The blastostyles, gonophores and gonothecae furnish a series of variations which can best be considered as so many stages of evolution.
Stage 1, seen inObelia. Numerous medusae are budded successively within the gonotheca and set free; they swim off and mature in the open sea (Allman [1], p. 48, figs. 18, 19).
Stage 2, seen inGonothyraea. Medusae, so-called “meconidia,” are budded but not liberated; each in turn, when it reaches sexual maturity, is protruded from the gonotheca by elongation of the stalk, and sets free the embryos, after which it withers and is replaced by another (Allman [1], p. 57, fig. 28).
Stage 3, seen inSertularia.—The gonophores are reduced in varying degree, it may be to sporosacs; they are budded successively from the blastostyle, and each in turn, when ripe, protrudes the spadix through the gonotheca (fig. 57, A, B). The spadix forms a gelatinous cyst, the so-called acrocyst (ac), external to the gonotheca (gth), enclosing and protecting the embryos. Then the spadix withers, leaving the embryos in the acrocyst, which may be further protected by a so-called marsupium, a structure formed by tentacle-like processes growing out from the blastostyle to enclose the acrocyst, each such process being covered by perisarc like a glove-finger secreted by it (fig. 57, C). (Allman [1], pp. 50, 51, figs. 21-24; Weismann [58], p. 170, pl. ix., figs. 7, 8.)
Stage 4, seen inPlumularidae.—The generative elements are produced in structures termed corbulae, formed by reduction and modification of branches of the colony. Each corbula contains a central row of blastostyles enclosed and protected by lateral rows of branches representing stunted buds (Allman [1], p. 66, fig. 30).
TheLeptomedusain form is generally shallow, more or less saucer-like, with velum less developed than in Anthomedusae (fig. 55). The characteristic sense-organs are ectodermal otocysts, absent, however, in some genera, in which case cordyli may replace them. When otocysts are present, they are at least eight in number, situated adradially, but are often very numerous. The cordyli are scattered on the ring-canal. Ocelli, if present, are borne on the tentacle-bulbs. The tentacles are usually hollow, rarely solid (Obelia). In number they are rarely less than four, but inDissonemathere are only two. Primitively there are four perradial tentacles, to which may be added four interradial, or they may become very numerous and are then scattered evenly round the margin, never arranged in tufts or clusters. In addition to tentacles, there may be marginal cirri (Laodice) with a solid endodermal axis, spirally coiled, very contractile, and bearing a terminal battery of nematocysts. The gonads are developed typically beneath the radial canals or below the stomach or its pouches, often stretching as long bands on to the base of the manubrium. InOctorchidae(fig. 58) each such band is interrupted, forming one mass at the base of the manubrium and another below the radial canal in each radius, in all eight separate gonad-masses, as the name implies. In some Leptomedusae excretory “marginal tubercles” are developed on the ring-canal.
Classification.—As in the Gymnoblastea, the difficulty of uniting the hydroid and medusan systems into one scheme of classification is very great in the present state of our knowledge. In a great many Leptomedusae the hydroid stage is as yet unknown, and it is by no means certain even that they possess one. It is quite possible that some of these medusae will be found to be truly hypogenetic, that is to say, with a life-cycle secondarily simplified by suppression of metagenesis. At present, ten recent and one extinct family of Calyptoblastea (Leptomedusae) may be recognized provisionally:1.Eucopidae(figs. 55, 59).—Trophosome with stalked hydrothecae; gonosome, free medusae with otocysts and four, rarely six or eight, unbranched radial canals. Two of the commonest British hydroids belong to this family,ObeliaandClytia.Obeliaforms numerous polyserial stems of the characteristic zigzag pattern growing up from a creeping basal stolon, and buds the medusa of the same name. InClytiathe polyps arise singly from the stolon, and the medusa is known asPhialidium(fig. 59).2.Aequoridae.—Trophosome only known in one genus (Polycanna), and similar to the preceding; gonosome, free medusae with otocysts and with at least eight radial canals, often a hundred or more, simple or branched.Aequoreais a common medusa.3.Thaumantidae.—Trophosome only known in one genus (Thaumantias), similar to that of theEucopidae; gonosome, free medusae with otocysts inconspicuous or absent, with usually four, sometimes eight, rarely more than eight, radial canals, simple and unbranched, along which the gonads are developed, with numerous tentacles bearing ocelli and with marginal sense-clubs.LaodiceandThaumantiasare representative genera.4.Berenicidae.—Trophosome unknown; gonosome, free medusae, with four or six radial canals, bearing the gonads, with numerous tentacles, between which occur sense-clubs, without otocysts.Berenice,Staurodiscus, &c.After Haeckel,System der Medusen, by permission of Gustav Fischer.Fig. 58.—Octorchandra canariensis, from life.5.Polyorchidae.—Trophosome unknown; gonosome, free medusae of deep form, with radial canals branched in a feathery manner, and bearing gonads on the main canal, but not on the branches, with numerous hollow tentacles bearing ocelli, and without otocysts.Polyorchis,Spirocodon.6.Campanularidae.--Trophosome as inEucopidae; gonosome, sessile gonophores. Many common or well-known genera belong here, such asHalecium,Campanularia,Gonothyraea, &c.7.Lafoëidae.—Trophosome as in the preceding; gonosome, free medusae or gonophores, the medusae with large open otocysts. The hydroid genusLafoëais remarkable for producing gonothecae on the hydrorhiza, each containing a blastostyle which bears a single gonophore; this portion of the colony was formerly regarded as an independent parasitic hydroid, and was namedCoppinia. Medusan genera areMitrocoma,Halopsis,Tiaropsis(fig. 29, &c.).(So far as the characters of the trophosome are concerned, the seven preceding families are scarcely distinguishable, and they form a section apart, contrasting sharply with the families next to be mentioned, in none of which are free medusae liberated from the colony, so that only the characters of the trophosome need be considered.)After E. T. Browne,Proc. Zool. Soc. of London, 1896.Fig. 59.—Three stages in the development ofPhialidium temporarium.a, The youngest stage, is magnified about 22 diam.;b, older, is magnified about 8 diam.;c, the adult medusa, is magnified.8.Sertularidae.—Hydrothecae sessile, biserial, alternating or opposite on the stem.SertulariaandSertularellaare two very common genera of this family.9.Plumularidae.—Hydrothecae sessile, biserial on the main stem, uniserial on the lateral branches or pinnules, which give the colony its characteristic feathery form; with nematophores. A very abundant and prolific family; well-known British genera arePlumularia,AntennulariaandAglaophenia.10.Hydroceratinidae.—This family contains the single Australian speciesClathrozoon wilsoniSpencer, in which a massive hydrorhizabears sessile hydrothecae, containing hydranths each with a single tentacle, and numerous nematophores. See W. B. Spencer [53].11.Dendrograptidae, containing fossil (Silurian) genera, such asDendrograptusandThamnograptus, of doubtful affinities.
Classification.—As in the Gymnoblastea, the difficulty of uniting the hydroid and medusan systems into one scheme of classification is very great in the present state of our knowledge. In a great many Leptomedusae the hydroid stage is as yet unknown, and it is by no means certain even that they possess one. It is quite possible that some of these medusae will be found to be truly hypogenetic, that is to say, with a life-cycle secondarily simplified by suppression of metagenesis. At present, ten recent and one extinct family of Calyptoblastea (Leptomedusae) may be recognized provisionally:
1.Eucopidae(figs. 55, 59).—Trophosome with stalked hydrothecae; gonosome, free medusae with otocysts and four, rarely six or eight, unbranched radial canals. Two of the commonest British hydroids belong to this family,ObeliaandClytia.Obeliaforms numerous polyserial stems of the characteristic zigzag pattern growing up from a creeping basal stolon, and buds the medusa of the same name. InClytiathe polyps arise singly from the stolon, and the medusa is known asPhialidium(fig. 59).
2.Aequoridae.—Trophosome only known in one genus (Polycanna), and similar to the preceding; gonosome, free medusae with otocysts and with at least eight radial canals, often a hundred or more, simple or branched.Aequoreais a common medusa.
3.Thaumantidae.—Trophosome only known in one genus (Thaumantias), similar to that of theEucopidae; gonosome, free medusae with otocysts inconspicuous or absent, with usually four, sometimes eight, rarely more than eight, radial canals, simple and unbranched, along which the gonads are developed, with numerous tentacles bearing ocelli and with marginal sense-clubs.LaodiceandThaumantiasare representative genera.
4.Berenicidae.—Trophosome unknown; gonosome, free medusae, with four or six radial canals, bearing the gonads, with numerous tentacles, between which occur sense-clubs, without otocysts.Berenice,Staurodiscus, &c.
5.Polyorchidae.—Trophosome unknown; gonosome, free medusae of deep form, with radial canals branched in a feathery manner, and bearing gonads on the main canal, but not on the branches, with numerous hollow tentacles bearing ocelli, and without otocysts.Polyorchis,Spirocodon.
6.Campanularidae.--Trophosome as inEucopidae; gonosome, sessile gonophores. Many common or well-known genera belong here, such asHalecium,Campanularia,Gonothyraea, &c.
7.Lafoëidae.—Trophosome as in the preceding; gonosome, free medusae or gonophores, the medusae with large open otocysts. The hydroid genusLafoëais remarkable for producing gonothecae on the hydrorhiza, each containing a blastostyle which bears a single gonophore; this portion of the colony was formerly regarded as an independent parasitic hydroid, and was namedCoppinia. Medusan genera areMitrocoma,Halopsis,Tiaropsis(fig. 29, &c.).
(So far as the characters of the trophosome are concerned, the seven preceding families are scarcely distinguishable, and they form a section apart, contrasting sharply with the families next to be mentioned, in none of which are free medusae liberated from the colony, so that only the characters of the trophosome need be considered.)
8.Sertularidae.—Hydrothecae sessile, biserial, alternating or opposite on the stem.SertulariaandSertularellaare two very common genera of this family.
9.Plumularidae.—Hydrothecae sessile, biserial on the main stem, uniserial on the lateral branches or pinnules, which give the colony its characteristic feathery form; with nematophores. A very abundant and prolific family; well-known British genera arePlumularia,AntennulariaandAglaophenia.
10.Hydroceratinidae.—This family contains the single Australian speciesClathrozoon wilsoniSpencer, in which a massive hydrorhizabears sessile hydrothecae, containing hydranths each with a single tentacle, and numerous nematophores. See W. B. Spencer [53].
11.Dendrograptidae, containing fossil (Silurian) genera, such asDendrograptusandThamnograptus, of doubtful affinities.
Order III.Hydrocorallinae.—Metagenetic colony-forming Hydromedusae, in which the polyp-colony forms a massive, calcareouscoralluminto which the polyps can be retracted; polyp-individuals always of two kinds, gastrozoids and dactylozoids; gonosome either free medusae or sessile gonophores. The trophosome consists of a mass of coenosarcal tubes anastomosing in all planes. The interspaces between the tubes are filled up by a solid mass of lime, consisting chiefly of calcium carbonate, which replaces the chitinous perisarc of ordinary hydroids and forms a stony corallum orcoenosteum(fig. 60). The surface of the coenosteum is covered by a layer of common ectoderm, containing large nematocysts, and is perforated by pores of two kinds, gastropores and dactylopores, giving exit to gastrozoids and dactylozoids respectively, which are lodged in vertical pore-canals of wider calibre than the coenosarcal canals of the general network. The coenosteum increases in size by new growth at the surface; and in the deeper, older portions of massive forms the tissues die off after a certain time, only the superficial region retaining its vitality down to a certain depth. The living tissues at the surface are cut off from the underlying dead portions by horizontal partitions termedtabulae, which are formed successively as the coenosteum increases in age and size. If the coenosteum ofMilleporabe broken across, each pore-canal (perhaps better termed a polyp-canal) is seen to be interrupted by a series of transverse partitions, representing successive periods of growth with separation from the underlying dead portions.
1,Sporadopora dichotoma.
2, 3,Allopora nobilis.
4,Allopora profunda.
5,Allopora miniacea.
6,Astylus subviridis.
7,Distichopora coccinea.
s, Style.
dp, Dactylopore.
gp, Gastropore.
b, In fig. 6, inner horseshoe-shaped mouth of gastropore.
Besides the wider vertical pore-canals and the narrower, irregular coenosarcal canals, the coenosteum may contain, in its superficial portion, chambers orampullae, in which the reproductive zoids (medusae or gonophores) are budded from the coenosarc.
The gastropores and dactylopores are arranged in various ways at the surface, a common pattern being the formation of a cyclosystem (fig. 60), in which a central gastrozoid is surrounded by a ring of dactylozoids (fig. 61). In such a system the dactylopores may be confluent with the gastropore, so that the entire cyclosystem presents itself as a single aperture subdivided by radiating partitions, thus having a superficial resemblance to a madreporarian coral with its radiating septa (figs. 62 and 63).
The gastrozoids usually bear short capitate tentacles, four, six or twelve in number; but inAstylus(fig. 63) they have no tentacles. The dactylozoids have no mouth; inMilleporidaethey have short capitate tentacles, but lack tentacles inStylasteridae.
The gonosome consists of free medusae inMilleporidae, which are budded from the apex of a dactylozoid inMillepora murrayi, but in other species from the coenosarcal canals. The medusae are produced by direct budding, without an entocodon in the bud. They are liberated in a mature condition, and probably live but a short time, merely sufficient to spread the species. The manubrium bearing the gonads is mouthless, and the umbrella is without tentacles, sense-organs, velum or radial canals. In theStylasteridaesessile gonophores are formed, always by budding from the coenosarc. InDistichoporathe gonophores have radial canals, but in other genera they are sporosacs with no trace of medusoid structure.
Classification.—Two families are known:—1.Milleporidae.—Coenosteum massive, irregular in form; pores scattered irregularly or in cyclosystems, without styles, with transverse tabulae; free medusae. A single genus,Millepora(figs. 60, 61).2.Stylasteridae.—Coenosteum arborescent, sometimes fanlike, with pores only on one face, or on the lateral margins of the branches; gastropores with tabulae only in two genera, but with (except inAstylus) astyle,i.e.a conical, thorn-like projection from the base of the pore, sometimes found also in dactylopores; sessile gonophores.Sporadoporahas the pores scattered irregularly.Distichoporahas the pores arranged in rows.Stylasterhas cyclosystems. InAlloporathe cyclostems resemble the calyces of Anthozoan corals. InCryptoheliathe cyclosystem is covered by a cap or operculum. InAstylus(fig. 63) styles are absent.Affinities of the Hydrocorallinae.—There can be no doubt that the forms comprised in this order bear a close relationship to the Hydroidea, especially the sub-order Gymnoblastea, with which they should perhaps be classed in a natural classification. A hydrocoralline may be regarded as a form of hydroid colony in which the coenosarc forms a felt-work ramifying in all planes, and in which the chitinous perisarc is replaced by a massive calcareous skeleton. So far as the trophosome is concerned, the step from an encrustinghydroid such asHydractiniato the hydrocorallineMilleporais not great.Hickson considers that the familiesMilleporidaeandStylasteridaeshould stand quite apart from one another and should not be united in one order. The nearest approach to theStylasteridaeis perhaps to be found inCeratella, with its arborescent trophosome formed of anastomosing coenosarcal tubes supported by a thick perisarc and covered by a common ectoderm.Ceratellastands in much the same relation to theStylasteridaethatHydractiniadoes to theMilleporidae, in both cases the chitinous perisarc being replaced by the solid coenosteum to which the hydrocorallines owe the second half of their name.
Classification.—Two families are known:—
1.Milleporidae.—Coenosteum massive, irregular in form; pores scattered irregularly or in cyclosystems, without styles, with transverse tabulae; free medusae. A single genus,Millepora(figs. 60, 61).
2.Stylasteridae.—Coenosteum arborescent, sometimes fanlike, with pores only on one face, or on the lateral margins of the branches; gastropores with tabulae only in two genera, but with (except inAstylus) astyle,i.e.a conical, thorn-like projection from the base of the pore, sometimes found also in dactylopores; sessile gonophores.Sporadoporahas the pores scattered irregularly.Distichoporahas the pores arranged in rows.Stylasterhas cyclosystems. InAlloporathe cyclostems resemble the calyces of Anthozoan corals. InCryptoheliathe cyclosystem is covered by a cap or operculum. InAstylus(fig. 63) styles are absent.
Affinities of the Hydrocorallinae.—There can be no doubt that the forms comprised in this order bear a close relationship to the Hydroidea, especially the sub-order Gymnoblastea, with which they should perhaps be classed in a natural classification. A hydrocoralline may be regarded as a form of hydroid colony in which the coenosarc forms a felt-work ramifying in all planes, and in which the chitinous perisarc is replaced by a massive calcareous skeleton. So far as the trophosome is concerned, the step from an encrustinghydroid such asHydractiniato the hydrocorallineMilleporais not great.
Hickson considers that the familiesMilleporidaeandStylasteridaeshould stand quite apart from one another and should not be united in one order. The nearest approach to theStylasteridaeis perhaps to be found inCeratella, with its arborescent trophosome formed of anastomosing coenosarcal tubes supported by a thick perisarc and covered by a common ectoderm.Ceratellastands in much the same relation to theStylasteridaethatHydractiniadoes to theMilleporidae, in both cases the chitinous perisarc being replaced by the solid coenosteum to which the hydrocorallines owe the second half of their name.
Order IV.Graptolitoidea(Rhabdophora, Allman).—This order has been constituted for a peculiar group of palaeozoic fossils, which have been interpreted as the remains of the skeletons of Hydrozoa of an extinct type.
A typical graptolite consists of an axis bearing a series of tooth-like projections, like a saw. Each such projection is regarded as representing a cup or hydrotheca, similar to those borne by a calyptoblastic hydroid, such asSertularia. The supposed hydrothecae may be present on one side of the axis only (monoprionid) or on both sides (diprionid); the first case may be conjectured to be the result of uniserial (helicoid) budding, the second to be produced by biserial (scorpioid) budding. In one division (Retiolitidae) the axis is reticulate. In addition to the stems bearing cups, there are found vesicles associated with them, which have been interpreted as gonothecae or as floats, that is to say, air-bladders, acting as hydrostatic organs for a floating polyp-colony.
Since no graptolites are known living, or, indeed, since palaeozoic times, the interpretation of their structure and affinities must of necessity be extremely conjectural, and it is by no means certain that they are Hydrozoa at all. It can only be said that their organization, so far as the state of their preservation permits it to be ascertained, offers closer analogies with the Hydrozoa, especially the Calyptoblastea, than with any other existing group of the animal kingdom.
See the treatise of Delage and Hérouard (Hydrozoa, [4]), and the articleGraptolites.
See the treatise of Delage and Hérouard (Hydrozoa, [4]), and the articleGraptolites.
Order V.Trachylinea.—Hydromedusae without alternation of generations,i.e.without a hydroid phase; the medusa develops directly from the actinula larva, which may, however, multiply by budding. Medusae with sense-organs represented by otocysts derived from modified tentacles (tentaculocysts), containing otoliths of endodermal origin, and innervated from the ex-umbral nerve-ring.
This order, containing the typical oceanic medusae, is divided into two sub-orders.
Sub-order 1. Trachomedusae.—Tentacles given off from the margin of the umbrella, which is entire,i.e.not lobed or indented; tentaculocysts usually enclosed in vesicles; gonads on the radial canals. The medusae of this order are characterized by the tough, rigid consistence of the umbrella, due partly to the dense nature of the mesogloea, partly to the presence of a marginal rim of chondral tissue, consisting of thickened ectoderm containing great numbers of nematocysts, and forming, as it were, a cushion-tyre supporting the edge of the umbrella. Prolongations from the rim of chondral tissue may form clasps orperoniasupporting the tentacles. The tentacles are primarily four in number, perradial, alternating with four interradial tentaculocysts, but both tentacles and sense-organs may be multiplied and the primary perradii may be six instead of four (fig. 26). The tentacles are always solid, containing an axis of endoderm-cells resembling notochordal tissue or plant-parenchyma, and are but moderately flexible. The sense-organs are tentaculocysts which are usually enclosed in vesicles and may be sunk far below the surface. The gonads are on the radial canals or on the stomach (Ptychogastridae), and each gonad may be divided into two by a longitudinal sub-umbral muscle-tract. The radial canals are four, six, eight or more, and in some genera blindly-ending centripetal canals are present (fig. 26). The stomach may be drawn out into the manubrium, forming a proboscis (“Magenstiel”) of considerable length.
The development of the Trachomedusae, so far as it is known, shows an actinula-stage which is either free (larval) or passed over in the egg (foetal) as inGeryonia; in no case does there appear to be a free planula-stage. The actinula, when free, may multiply by larval budding, but in all cases both the original actinula and all its descendants become converted into medusae, so that there is no alternation of generations. InGonionemusthe actinula becomes attached and polyp-like and reproduces by budding.
The Trachomedusae are divided into the following families:1.Petasidae(Petachnidae).—Four radial canals, four gonads; stomach not prolonged into the manubrium, which is relatively short; tentaculocysts free.Petasusand other genera make up this family, founded by Haeckel, but no other naturalist has ever seen them, and it is probable that they are simply immature forms of other genera.2.Olindiadae, with four radial canals and four gonads; manubrium short; ring-canals giving off blind centripetal canals; tentaculocysts enclosed.Olindias mülleri(fig. 64) is a common Mediterranean species. Other genera areAglauropsis,GosseaandGonionemus; the last named bears adhesive suckers on the tentacles. Some doubt attaches to the position of this family. It has been asserted that the tentaculocysts are entirely ectodermal and that either the family should be placed amongst the Leptomedusae, or should form, together with certain Leptomedusae, an entirely distinct order. InGonionemus, however, the concrement-cells are endodermal.3.Trachynemidae.—Eight radial canals, eight gonads, stomach not prolonged into manubrium; tentaculocysts enclosed.Rhopalonema,Trachynema, &c.After E. T. Browne,Proc. Zool. Soc. of London.Fig. 65.—Aglantha rosea(Forbes), a British medusa.4.Ptychogastridae(Pectyllidae).—As in the preceding, but with suckers on the tentacles.PtychogastriaAllman (=Pectyllis), a deep-sea form.5.Aglauridae.—Eight radial canals, two, four or eight gonads; tentacles numerous; tentaculocysts free; stomach prolonged into manubrium.Aglaura,Aglantha(fig. 65), &c., with eight gonads;Stauraglaurawith four;Persawith two.Amphogona, hermaphrodite, with male and female gonads on alternating radial canals.6.Geryonidae.—Four or six radial canals; gonads band-like; stomach prolonged into a manubrium of great length; tentaculocysts enclosed.Liriope, &c., with four radial canals;Geryonia,Carmarina(fig. 26), &c., with six.7.Halicreidae.—Eight very broad radial canals; ex-umbrella often provided with lateral outgrowths; tentacles differing in size, but in a single row.Halicreas.
The Trachomedusae are divided into the following families:
1.Petasidae(Petachnidae).—Four radial canals, four gonads; stomach not prolonged into the manubrium, which is relatively short; tentaculocysts free.Petasusand other genera make up this family, founded by Haeckel, but no other naturalist has ever seen them, and it is probable that they are simply immature forms of other genera.
2.Olindiadae, with four radial canals and four gonads; manubrium short; ring-canals giving off blind centripetal canals; tentaculocysts enclosed.Olindias mülleri(fig. 64) is a common Mediterranean species. Other genera areAglauropsis,GosseaandGonionemus; the last named bears adhesive suckers on the tentacles. Some doubt attaches to the position of this family. It has been asserted that the tentaculocysts are entirely ectodermal and that either the family should be placed amongst the Leptomedusae, or should form, together with certain Leptomedusae, an entirely distinct order. InGonionemus, however, the concrement-cells are endodermal.
3.Trachynemidae.—Eight radial canals, eight gonads, stomach not prolonged into manubrium; tentaculocysts enclosed.Rhopalonema,Trachynema, &c.
4.Ptychogastridae(Pectyllidae).—As in the preceding, but with suckers on the tentacles.PtychogastriaAllman (=Pectyllis), a deep-sea form.
5.Aglauridae.—Eight radial canals, two, four or eight gonads; tentacles numerous; tentaculocysts free; stomach prolonged into manubrium.Aglaura,Aglantha(fig. 65), &c., with eight gonads;Stauraglaurawith four;Persawith two.Amphogona, hermaphrodite, with male and female gonads on alternating radial canals.
6.Geryonidae.—Four or six radial canals; gonads band-like; stomach prolonged into a manubrium of great length; tentaculocysts enclosed.Liriope, &c., with four radial canals;Geryonia,Carmarina(fig. 26), &c., with six.
7.Halicreidae.—Eight very broad radial canals; ex-umbrella often provided with lateral outgrowths; tentacles differing in size, but in a single row.Halicreas.
Sub-order 2. Narcomedusae.—Margin of the umbrella-lobed, tentacles arising from the ex-umbrella at some distance from the margin; tentaculocysts exposed, not enclosed in vesicles; gonads on the sub-umbral floor of the stomach or of the gastric pouches.
c, Circular canal.
h, “Otoporpae” or centripetal process of the marginal cartilaginous ring connected with tentaculocyst.
k, Stomach.
l, Jelly of the disk.
r, Radiating canal (pouch of stomach).
tt, Tentacles.
tw, Tentacle root.
The Narcomedusae exhibit peculiarities of form and structure which distinguish them at once from all other Hydromedusae. The umbrella is shallow and has the margin supported by a rim of thickened ectoderm, as in the Trachomedusae, but not so strongly developed. The tentacles are not inserted on the margin of the umbrella, but arise high up on the ex-umbral surface, and the umbrella is prolonged into lobes corresponding to the interspaces between the tentacles. The condition of things can be imagined by supposing that in a medusa primitively of normal build, with tentacles at the margin, the umbrella has grown down past the insertion of the tentacles. As a result of this extension of the umbrellar margin, all structures belonging to this region, namely, the ring-canal, the nerve-rings, and the rim of thickened ectoderm, do not run an even course, but are thrown into festoons, caught up under the insertion of each tentacle in such a way that the ring-canal and its accompaniments form in each notch of the umbrellar margin an inverted V, the apex of which corresponds to the insertion of the tentacle; in some cases the limbs of the V may run for some distance parallel to one another, and may be fused into one, giving a figure better compared to an inverted Y. Thus the ectodermal rim runs round the edge of each lobe of the umbrella and then passes upwards towards the base of the tentacle from the re-entering angle between two adjacent lobes, to form with its fellow of the next lobe a tentacle-clasp orperonium,i.e.a streak of thickened ectoderm supporting the tentacle. Similarly the ring-canal runs round the edge of the lobe as the so-called festoon-canal, and then runs upwards under the peronium to the base of the tentacle as one of a pair of peronial canals, the limbs of the V-like figure already mentioned. The nerve-rings have a similar course. The tentaculocysts are implanted round the margins of the lobes of the umbrella and may be supported by prolongations of the ectodermal rim termedotoporpae(Gehörspangen). The radial canals are represented by wide gastric pouches, and may be absent, so that the tentacles arise directly from the stomach (Solmaridae). The tentacles are always solid, as in Trachomedusae.
The development of the Narcomedusae is in the main similar to that of the Trachomedusae, but shows some remarkable features. InAeginopsisa planula is formed by multipolar immigration. The two ends of the planula become greatly lengthened and give rise to the two primary tentacles of the actinula, of which the mouth arises from one side of the planula. Hence the principal axis of the future medusa corresponds, not to the longitudinal axis of the planula, but to a transverse axis. This is in some degree parallel to the cases described above, in which a planula gives rise to the hydrorhiza, and buds a polyp laterally.
InCuninaand allied genera the actinula, formed in the manner described, has a hypostome of great length, quite disproportionate to the size of the body, and is further endowed with the power of producing buds from a stolon arising from the aboral side of the body. In these species the actinula is parasitic upon another medusa; for instance,Cunoctantha octonariauponTurritopsis,C. proboscideauponLiriopeorGeryonia. The parasite effects a lodgment in the host either by invading it as a free-swimming planula, or, apparently, in other cases, as a spore-embryo which is captured and swallowed as food by the host. The parasitic actinula is found attached to the proboscis of the medusa; it thrusts its greatly elongated hypostome into the mouth of the medusa and nourishes itself upon the food in the digestive cavity of its host. At the same time it produces buds from an aboral stolon. The buds become medusae by the direct method of budding described above. In some cases the buds do not become detached at once, but the stolon continues to grow and to produce more buds, forming a “bud-spike” (Knospenähre), which consists of the axial stolon bearing medusa-buds in all stages of development. In such cases the original parent-actinula does not itself become a medusa, but remains arrested in development and ultimately dies off, so that a true alternation of generations is brought about. It is in these parasitic forms that we meet with the method of reproduction by sporogony described above.
In other Narcomedusae,e.g.Cunoctantha fowleriBrowne, buds are formed from the sub-umbrella on the under side of the stomach pouches, where later the gonads are developed.
Classification.—Three families of Narcomedusae are recognized (see O. Maas [40]):After O. Maas,Craspedoten Medusen der Siboga Expedition, by permission of E. S. Brill & Co.Fig. 67.—Solmundella bitentaculata(Quoy and Gaimard).1.Cunanthidae.—With broad gastric pouches which are simple,i.e.undivided, and “pernemal,”i.e.correspond in position with the tentacles.Cunina(fig. 66) with more than eight tentacles;Cunoctanthawith eight tentacles, four perradial, four interradial.2.Aeginidae.—Radii a multiple of four, with radial gastric pouches bifurcated or subdivided; the tentacles are implanted in the notch between the two subdivisions of each (primary) gastric pouch, hence the (secondary) gastric pouches appear to be “internemal” in position,i.e.to alternate in position with the tentacles.Aegina, with four tentacles and eight pouches;Aeginura(fig. 25), with eight tentacles and sixteen pouches;Solmundella(fig. 67), with two tentacles and eight pouches;Aeginopsis(fig. 23), with two or four tentacles and sixteen pouches.3.Solmaridae.—No gastric pouches; the numerous tentacles arise direct from the stomach, into which also the peronial canals open, so that the ring-canal is cut up into separate festoons.Solmaris,Pegantha,Polyxenia, &c. To this family should be referred, probably, the genusHydroctena, described by C. Dawydov [11a] and regarded by him as intermediate between Hydromedusae and Ctenophora. See O. Maas [35].Appendix to the Trachylinae.Of doubtful position, but commonly referred to the Trachylinae, are the two genera of fresh-water medusae,LimnocodiumandLimnocnida.Limnocodium sowerbyiwas first discovered in theVictoria regiatank in the Botanic Gardens, Regent’s Park, London. Since then it has been discovered in other botanic gardens in various parts of Europe, its two most recent appearances being at Lyons (1901) and Munich (1905), occurring always in tanks in which theVictoria regiais cultivated, a fact which indicates that tropical South America is its original habitat. In the same tanks a small hydroid, very similar toMicrohydra, has been found, which bears medusa-buds and is probably the stock from which the medusa is budded. It is a remarkable fact that all specimens ofLimnocodiumhitherto seen have been males; it may be inferred from this either that only one polyp-stock has been introduced into Europe, from which all the medusae seen hitherto have been budded, or perhaps that the female medusa is a sessile gonophore, as inPennaria. The male gonads are carried on the radial canals.Limnocnida tanganyicaewas discovered first in Lake Tanganyika, but has since been discovered also in Lake Victoria and in the river Niger. It differs fromLimnocodiumin having practically no manubrium but a wide mouth two-thirds the diameter of the umbrella across. It buds medusae from the margin of the mouth in May and June, and in August and September the gonads are formed in the place where the buds arose. The hydroid phase, if any, is not known.Both these medusae have sense-organs of a peculiar type, which are said to contain an endodermal axis like the sense-organs of Trachylinae, but the fact has recently been called in question forLimnocodiumby S. Goto, who considers the genus to be allied toOlindias. Allman, on the other hand, referredLimnocodiumto the Leptomedusae.In this connexion must be mentioned, finally, the medusae budded from the fresh-water polypMicrohydra. The polyp-stages ofLimnocodiumandMicrohydraare extremely similar in character. In both cases the hydranth is extremely reduced and has no tentacles, and the polyp forms a colony by budding from the base. InLimnocodiumthe body secretes a gelatinous mucus to which adhere particles of mud, &c., forming a protective covering. InMicrohydrano such protecting case is formed. In view of the great resemblance betweenMicrohydraand the polyp ofLimnocodium, it might be expected that the medusae to which they give origin would also be similar. As yet, however, the medusa ofMicrohydrahas only been seen in an immature condition, but it shows some well-marked differences fromLimnocodium, especially in the structure of the tentacles, which furnish useful characters for distinguishing species amongst medusae. The possession of a polyp-stage byLimnocodiumandMicrohydrafurnishes an argument against placing them in the Trachylinae. Their sense-organs require renewed investigations. (Browne [10] and [10a].)
Classification.—Three families of Narcomedusae are recognized (see O. Maas [40]):
1.Cunanthidae.—With broad gastric pouches which are simple,i.e.undivided, and “pernemal,”i.e.correspond in position with the tentacles.Cunina(fig. 66) with more than eight tentacles;Cunoctanthawith eight tentacles, four perradial, four interradial.
2.Aeginidae.—Radii a multiple of four, with radial gastric pouches bifurcated or subdivided; the tentacles are implanted in the notch between the two subdivisions of each (primary) gastric pouch, hence the (secondary) gastric pouches appear to be “internemal” in position,i.e.to alternate in position with the tentacles.Aegina, with four tentacles and eight pouches;Aeginura(fig. 25), with eight tentacles and sixteen pouches;Solmundella(fig. 67), with two tentacles and eight pouches;Aeginopsis(fig. 23), with two or four tentacles and sixteen pouches.
3.Solmaridae.—No gastric pouches; the numerous tentacles arise direct from the stomach, into which also the peronial canals open, so that the ring-canal is cut up into separate festoons.Solmaris,Pegantha,Polyxenia, &c. To this family should be referred, probably, the genusHydroctena, described by C. Dawydov [11a] and regarded by him as intermediate between Hydromedusae and Ctenophora. See O. Maas [35].
Appendix to the Trachylinae.
Of doubtful position, but commonly referred to the Trachylinae, are the two genera of fresh-water medusae,LimnocodiumandLimnocnida.
Limnocodium sowerbyiwas first discovered in theVictoria regiatank in the Botanic Gardens, Regent’s Park, London. Since then it has been discovered in other botanic gardens in various parts of Europe, its two most recent appearances being at Lyons (1901) and Munich (1905), occurring always in tanks in which theVictoria regiais cultivated, a fact which indicates that tropical South America is its original habitat. In the same tanks a small hydroid, very similar toMicrohydra, has been found, which bears medusa-buds and is probably the stock from which the medusa is budded. It is a remarkable fact that all specimens ofLimnocodiumhitherto seen have been males; it may be inferred from this either that only one polyp-stock has been introduced into Europe, from which all the medusae seen hitherto have been budded, or perhaps that the female medusa is a sessile gonophore, as inPennaria. The male gonads are carried on the radial canals.
Limnocnida tanganyicaewas discovered first in Lake Tanganyika, but has since been discovered also in Lake Victoria and in the river Niger. It differs fromLimnocodiumin having practically no manubrium but a wide mouth two-thirds the diameter of the umbrella across. It buds medusae from the margin of the mouth in May and June, and in August and September the gonads are formed in the place where the buds arose. The hydroid phase, if any, is not known.
Both these medusae have sense-organs of a peculiar type, which are said to contain an endodermal axis like the sense-organs of Trachylinae, but the fact has recently been called in question forLimnocodiumby S. Goto, who considers the genus to be allied toOlindias. Allman, on the other hand, referredLimnocodiumto the Leptomedusae.
In this connexion must be mentioned, finally, the medusae budded from the fresh-water polypMicrohydra. The polyp-stages ofLimnocodiumandMicrohydraare extremely similar in character. In both cases the hydranth is extremely reduced and has no tentacles, and the polyp forms a colony by budding from the base. InLimnocodiumthe body secretes a gelatinous mucus to which adhere particles of mud, &c., forming a protective covering. InMicrohydrano such protecting case is formed. In view of the great resemblance betweenMicrohydraand the polyp ofLimnocodium, it might be expected that the medusae to which they give origin would also be similar. As yet, however, the medusa ofMicrohydrahas only been seen in an immature condition, but it shows some well-marked differences fromLimnocodium, especially in the structure of the tentacles, which furnish useful characters for distinguishing species amongst medusae. The possession of a polyp-stage byLimnocodiumandMicrohydrafurnishes an argument against placing them in the Trachylinae. Their sense-organs require renewed investigations. (Browne [10] and [10a].)
Order VI.Siphonophora.—Pelagic floating Hydrozoa with great differentiation of parts, each performing a special function; generally regarded as colonies showing differentiation of individuals in correspondence with a physiological division of labour.
n, Pneumatocyst.
k, Nectocalyces (swimming bells).
l, Hydrophyllium (covering-piece).
i, Generative medusiform person.
g, Palpon with attached palpacle,h.
e, Siphon with branched grappling tentacle,f.
m, Stem.
A typical Siphonophore is a stock orcormusconsisting of a number ofappendagesplaced in organic connexion with one another by means of acoenosarc. The coenosarc does not differ in structure from that already described in colonial Hydrozoa. It consists of a hollow tube, or tubes, of which the wall is made up of the two body-layers, ectoderm and endoderm, and the cavity is a continuation of the digestive cavities of the nutritive and other appendages,i.e.of the coelenteron. The coenosarc may consist of a single elongated tube or stolon, forming the stem or axis of the cormus on which, usually, the appendages are arranged in groups termed cormidia; or it may take the form of a compact mass of ramifying, anastomosing tubes, in which case the cormus as a whole has a compact form andcormidiaare not distinguishable. In the Disconectae the coenosarc forms a spongy mass, the “centradenia,” which is partly hepatic in function, forming the so-called liver, and partly excretory.
The appendages show various types of form and structure corresponding to different functions. The cormus is always differentiated into two parts; an upper portion termed thenectosome, in which the appendages are locomotor or hydrostatic in function, that is to say, serve for swimming or floating; and a lower portion termed thesiphosome, bearing appendages which are nutritive, reproductive or simply protective in function.
Divergent views have been held by different authors both as regards the nature of the cormus as a whole, and as regards the homologies of the different types of appendages borne by it.
The general theories of Siphonophoran morphology are discussed below, but in enumerating the various types of appendages it is convenient to discuss their morphological interpretation at the same time.After A. Agassiz, from Lankester’sTreatise on Zoology.Fig. 69.—Porpita, seen from above, showing the pneumatophore and expanded palpons.In the nectosome one or more of the following types of appendage occur:—1. Swimming-bells, termednectocalycesornectophores(fig. 68, k), absent inChondrophoridaandCystophorida; they are contractile and resemble, both in appearance, structure and function, the umbrella of a medusa, with radial canals, ring-canal and velum; but they are without manubrium, tentacles or sense-organs, and are always bilaterally symmetrical, a peculiarity of form related with the fact that they are attached on one side to the stem. A given cormus may bear one or several nectocalyces, and by their contractions they propel the colony slowly along, like so many medusae harnessed together. In cases where the cormus has no pneumatophore the topmost swimming bell may contain an oil-reservoir oroleocyst.2. The pneumatophore or air-bladder (fig. 68,n), for passive locomotion, forming a float which keeps the cormus at or near the surface of the water. The pneumatophore arises from the ectoderm as a pit or invagination, part of which forms a gas-secreting gland, while the rest gives rise to an air-sack lined by a chitinous cuticle. The orifice of invagination forms a pore which may be closed up or may form a protruding duct or funnel. As in the analogous swim-bladder of fishes, the gas in the pneumatophore can be secreted or absorbed, whereby the specific gravity of the body can be diminished or increased, so as to cause it to float nearer the surface or at a deeper level. Never more than one pneumatophore is found in a cormus, and when present it is always situated at the highest point above the swimming bells, if these are present also. InVelellathe pneumatophore becomes of complex structure and sends air-tubes, lined by a chitin and resembling tracheae, down into the compact coenosarc, thus evidently serving a respiratory as well as a hydrostatic function.Divergent views have been held as to the morphological significance of the pneumatophore. E. Haeckel regarded the whole structure as a glandular ectodermal pit formed on the ex-umbral surface of a medusa-person. C. Chun and, more recently, R. Woltereck [59], on the other hand, have shown that the ectodermal pit which gives rise to the pneumatophore represents an entocodon. Hence the cavity of the air-sack is equivalent to a sub-umbral cavity in which no manubrium is formed, and the pore or orifice of invagination would represent the margin of the umbrella. In the wall of the sack is a double layer of endoderm, the space between which is a continuation of the coelenteron. By coalescence of the endoderm-layers, the coelenteron may be reduced to vessels, usually eight in number, opening into a ring-sinus surrounding the pore. Thus the disposition of the endoderm-cavities is roughly comparable to the gastrovascular system of a medusa.The difference between the theories of Haeckel and Chun is connected with a further divergence in the interpretation of the stem or axis of the cormus. Haeckel regards it as the equivalent of the manubrium, and as it is implanted on the blind end of the pneumatophore, such a view leads necessarily to the air-sack and gland being a development on the ex-umbral surface of the medusa-person. Chun and Woltereck, on the other hand, regard the stem as astolo proliferarising from the aboral pole, that is to say, from the ex-umbrella, similar to that which grows out from the ex-umbral surface of the embryo of the Narcomedusae and produces buds, a view which is certainly supported by the embryological evidence to be adduced shortly.In the siphosome the following types of appendages occur:—1.Siphonsor nutritive appendages, from which the order takes its name; never absent and usually present in great numbers (fig. 68, e). Each is a tube dilated at or towards the base and containing a mouth at its extremity, leading into a stomach placed in the dilatation already mentioned. The siphons have been compared to the manubrium of a medusa-individual, or to polyps, and hence are sometimes termed gastrozoids.2.Palpons(fig. 68,g), present in some genera, especially in Physonectae; similar to the siphons but without a mouth, and purely tactile in function, hence sometimes termed dactylozoids. If a distal pore or aperture is present, it is excretory in function; such varieties have been termed “cystons” by Haeckel.3.Tentacles(“Fangfäden”), always present, and implanted one at the base of each siphon (fig. 68,f). The tentacles of siphonophores may reach a great length and have a complex structure. They may bear accessory filaments ortentilla(f′), covered thickly with batteries of nematocysts, to which these organisms owe their great powers of offence and defence.4.Palpacles(“Tastfäden”), occurring together with palpons, one implanted at the base of each palpon (fig. 68,h). Each palpacle is a tactile filament, very extensile, without accessory filaments or nematocysts.5.Bracts(“hydrophyllia”), occur inCalycophoridaand somePhysophoridaas scale-like appendages protecting other parts (fig. 68,l). The mesogloea is greatly developed in them and they are often of very tough consistency. By Haeckel they are considered homologous with the umbrella of a medusa.From G. H. Fowler, after A. Agassiz, Lankester’sTreatise on Zoology.Fig. 70.—Diagram of the structure ofVelella, showing the central and peripheral thirds of a half-section of the colony, the middle third being omitted. The ectoderm is indicated by close hatching, the endoderm by light hatching, the mesogloea by thick black lines, the horny skeleton of the pneumatophore and sail by dotting.BL, Blastostyle.C, Centradenia.D, Palpon.EC, Edge of colony prolonged beyond the pneumatophore.G, Cavity of the large central siphon.M, Medusoid gonophores.PN, Primary central chamber, and PN′, concentric chamber of the pneumatophore, showing an opening to the exterior and a “trachea.”S, Sail.6.Gonostyles, appendages which produce by budding medusae or gonophores, like the blastostyles of a hydroid colony. In their most primitive form they are seen inVelellaas “gonosiphons,” which possess mouths like the ordinary sterile siphons and bud free medusae. In other forms they have no mouths. They may be branched, so-called “gonodendra,” and amongst them may occur special forms of palpons, “gonopalpons.” The gonostyles have been compared to the blastostyles of a hydroid colony, or to the manubrium of a medusa which produces free or sessile medusa-buds.7.Gonophores, produced either on the gonostyles already mentioned or budded, as in hydrocorallines, from the coenosarc,i.e.the stem (fig. 68,i.). They show every transition between free medusae and sporosacs, as already described, for hydroid colonies. Thus inVelellafree medusae are produced, which have been described as an independent genus of medusae,Chrysomitra. In other types the medusae may be set free in a mature condition as the so-called “genital swimming bells,” comparable to theGlobiceps of Pennaria. The most usual condition, however, is that in which sessile medusoid gonophores or sporosacs are produced.From G. H. Fowler, after G. Cuvier, Lankester’sTreatise on Zoology.Fig. 71.—Upper surface ofVelella, showing pneumatophore and sail.The various types of appendages described in the foregoing may be arranged in groups termedcormidia. In forms with a compact coenosarc such asVelella,Physalia, &c., the separate cormidia cannot be sharply distinguished, and such a condition is described technically as one with “scattered” cormidia. In forms in which, on the other hand, the coenosarc forms an elongated, tubular axis or stem, the appendages are arranged as regularly recurrent cormidia along it, and the cormidia are then said to be “ordinate.” In such cases the oldest cormidia, that is to say, those furthest from the nectosome, may become detached (like the segments or proglottides of a tape-worm) and swim off, each such detached cormidium then becoming a small free cormus which, in many cases, has been given an independent generic name. A cormidium may contain a single nutritive siphon (“monogastric”) or several siphons (“polygastric”):The following are some of the forms of cormidia that occur:—1. Theeudoxome(Calycophorida), consisting of a bract, siphon, tentacle and gonophore; when free it is known asEudoxia.2. Theersaeome(Calycophorida), made up of the same appendages as the preceding type but with the addition of a nectocalyx; when free termedErsaea.3. Therhodalomeof someRhodalidae, consisting of siphon, tentacle and one or more gonophores.4. TheathoromeofPhysophora, &c., consisting of siphon, tentacle, one or more palpons with palpacles, and one or more gonophores.5. ThecrystallomeofAnthemodes, &c., similar to the athorome but with the addition of a group of bracts.Fig. 72.—A,Diphyes campanulata; B, a group of appendages (cormidium) of the sameDiphyes. (After C. Gegenbaur.)a, Axis of the colony.m, Nectocalyx.c, Sub-umbral cavity of nectocalyx.v, Radial canals of nectocalyx.o, Orifice of nectocalyx.t, Bract.n, Siphon.g, Gonophore.i, Tentacle.Embryology of the Siphonophora.—The fertilized ovum gives rise to a parenchymula, with solid endoderm, which is set free as a free-swimming planula larva, in the manner already described (seeHydrozoa). The planula has its two extremities dissimilar (Bipolaria-larva). The subsequent development is slightly different according as the future cormus is headed by a pneumatophore (Physophorida, Cystophorida) or by a nectocalyx (Calycophorida).(i.) Physophorida, for exampleHalistemma(C. Chun,Hydrozoa[1]). The planula becomes elongated and broader towards one pole, at which a pit or invagination of the ectoderm arises. Next the pit closes up to form a vesicle with a pore, and so gives rise to the pneumatophore. From the broader portion of the planula an outgrowth arises which becomes the first tentacle of the cormus. The endoderm of the planula now acquires a cavity, and at the narrower pole a mouth is formed, giving rise to the primary siphon. Thus from the original planula three appendages are, as it were, budded off, while the planula itself mostly gives rise to coenosarc, just as in some hydroids the planula is converted chiefly into hydrorhiza.(ii.) Calycophorida, for example,Muggiaea. The planula develops, on the whole, in a similar manner, but the ectodermal invagination arises, not at the pole of the planula, but on the side of its broader portion, and gives rise, not to a pneumatophore, but to a nectocalyx, the primary swimming bell orprotocodon(“Fallschirm”) which is later thrown off and replaced by secondary swimming bells,metacodons, budded from the coenosarc.From a comparison of the two embryological types there can be no doubt on two points; first, that the pneumatophore and the protocodon are strictly homologous, and, therefore if the nectocalyx is comparable to the umbrella of a medusa, as seems obvious, the pneumatophore must be so too; secondly, that the coenosarcal axis arises from the ex-umbrella of the medusa and cannot be compared to a manubrium, but is strictly comparable to the “bud-spike” of a Narcomedusan.Theories of Siphonophore Morphology.—The many theories that have been put forward as to the interpretation of the cormus and the various parts are set forth and discussed in the treatise of Y. Delage and E. Hérouard (Hydrozoa[4]) and more recently by R. Woltereck [59], and only a brief analysis can be given here.After C. Gegenbaur.Fig. 73.—Physophora hydrostatica.a′, Pneumatocyst.t, Palpons.a, Axis of the colony.m, Nectocalyx.o, Orifice of nectocalyx.n, Siphon.g, Gonophore.i, Tentacle.In the first place the cormus has been regarded as a single individual and its appendages asorgans. This is the so-called “polyorgan” theory, especially connected with the name of Huxley; but it must be borne in mind that Huxley regarded all the forms produced, in any animal, between one egg-generation and the next, as constituting in the lump one single individual. Huxley, therefore, considered a hydroid colony, for example, as a single individual, and each separate polyp or medusa budded from it as having the value of an organ and not of an individual. Hence Huxley’s view is not so different from those held by other authors as it seems to be at first sight.In more recent years Woltereck [59] has supported Huxley’s view of individuality, at the same time drawing a fine distinction between “individual” and “person.” The individual is the product of sexual reproduction; a person is an individual of lower rank, which may be produced asexually. A Siphonophore is regarded as a single individual composed of numerous zoids, budded from the primary zoid (siphon) produced from the planula. Any given zoid is a person-zoid if equivalent to the primary zoid, an organ-zoid if equivalent only to a part of it. Woltereck considers the siphonophores most nearly allied to the Narcomedusae, producing like the buds from an aboral stolon, the first bud being represented by the pneumatophore or protocodon, in different cases.Contrasting, in the second place, with the polyorgan theory are the various “polyperson” theories which interpret the Siphonophore cormus as a colony composed of more or fewer individuals in organic union with one another. On this interpretation there is still room for considerable divergence of opinion as regards detail. To begin with, it is not necessary on the polyperson theory to regard each appendage as a distinct individual; it is still possible to compare appendages with parts of an individual which have become separated from one another by a process of “dislocation of organs.” Thus a bract may be regarded, with Haeckel, as a modified umbrella of a medusa, a siphon as its manubrium, and a tentacle as representing a medusan tentacle shifted in attachment from the margin to the sub-umbrella; or a siphon may be compared with a polyp, of which the single tentacle has become shifted so as to be attached to the coenosarc and so on. Some authors prefer, on the other hand, to regard every appendage as a separate individual, or at least as a portion of an individual, of which other portions have been lost or obliterated.A further divergence of opinion arises from differences in the interpretation of the persons composing the colony. It is possible to regard the cormus (1) as a colony of medusa-persons, (2) as a colony of polyp-persons, (3) as composed partly of one, partly of the other. It is sufficient here to mention briefly the views put forward on this point by C. Chun and R. Haeckel.Chun (Hydrozoa[1]) maintains the older views of Leuckart and Claus, according to which the cormus is to be compared to a floating hydroid colony. It may be regarded as derived from floating polyps similar toNemopsisorPelagohydra, which by budding produce a colony of polyps and also form medusa-buds. The polyp-individuals form the nutritive siphosome or trophosome. The medusa-buds are either fertile or sterile. If fertile they become free medusae or sessile gonophores. If sterile they remain attached and locomotor in function, forming the nectosome, the pneumatophore and swimming-bells.Haeckel, on the other hand, is in accordance with Balfour in regarding a Siphonophore as a medusome, that is to say, as a colony composed of medusoid persons or organs entirely. Haeckel considers that the Siphonophores have two distinct ancestral lines of evolution:1. In theDisconanthae,i.e.in such forms asVelella,Porpita, &c., the ancestor was an eight-rayed medusa (Disconula) which acquired a pneumatophore as an ectodermal pit on the ex-umbrella, and in which the organs (manubrium, tentacles, &c.) became secondarily multiplied, just as they do inGastroblastaas the result of incomplete fission. The nearest living allies of the ancestralDisconulaare to be sought in thePectyllidae.After Haeckel, from Lankester’sTreatise on Zoology.Fig. 74.—Stephalia corona, a young colony.p, Pneumatophore.n, Nectocalyx.l, Aurophore.lo, Orifice of the aurophore.s, Siphon.t, Tentacle.2. In theSiphonanthae,i.e.in all other Siphonophores, the ancestral form was aSiphonula, a bilaterally symmetrical Anthomedusa with a single long tentacle (cf.Corymorpha), which became displaced from the margin to the sub-umbrella. TheSiphonulaproduced buds on the manubrium, as many Anthomedusae are known to do, and these by reduction or dislocation of parts gave rise to the various appendages of the colony. Thus the umbrella of theSiphonulabecame the protocodon, and its manubrium, the axis or stolon, which, by a process of dislocation of organs, escaped, as it were, from the sub-umbrella through a cleft and became secondarily attached to the ex-umbrella. It must be pointed out that, however probable Haeckel’s theory may be in other respects, there is not the slightest evidence for any such cleft in the umbrella having been present at any time, and that the embryological evidence, as already pointed out, is all against any homology between the stem and a manubrium, since the primary siphon does not become the stem, which arises from the ex-umbral side of the protocodon and is strictly comparable to a stolon.
The general theories of Siphonophoran morphology are discussed below, but in enumerating the various types of appendages it is convenient to discuss their morphological interpretation at the same time.
In the nectosome one or more of the following types of appendage occur:—
1. Swimming-bells, termednectocalycesornectophores(fig. 68, k), absent inChondrophoridaandCystophorida; they are contractile and resemble, both in appearance, structure and function, the umbrella of a medusa, with radial canals, ring-canal and velum; but they are without manubrium, tentacles or sense-organs, and are always bilaterally symmetrical, a peculiarity of form related with the fact that they are attached on one side to the stem. A given cormus may bear one or several nectocalyces, and by their contractions they propel the colony slowly along, like so many medusae harnessed together. In cases where the cormus has no pneumatophore the topmost swimming bell may contain an oil-reservoir oroleocyst.
2. The pneumatophore or air-bladder (fig. 68,n), for passive locomotion, forming a float which keeps the cormus at or near the surface of the water. The pneumatophore arises from the ectoderm as a pit or invagination, part of which forms a gas-secreting gland, while the rest gives rise to an air-sack lined by a chitinous cuticle. The orifice of invagination forms a pore which may be closed up or may form a protruding duct or funnel. As in the analogous swim-bladder of fishes, the gas in the pneumatophore can be secreted or absorbed, whereby the specific gravity of the body can be diminished or increased, so as to cause it to float nearer the surface or at a deeper level. Never more than one pneumatophore is found in a cormus, and when present it is always situated at the highest point above the swimming bells, if these are present also. InVelellathe pneumatophore becomes of complex structure and sends air-tubes, lined by a chitin and resembling tracheae, down into the compact coenosarc, thus evidently serving a respiratory as well as a hydrostatic function.
Divergent views have been held as to the morphological significance of the pneumatophore. E. Haeckel regarded the whole structure as a glandular ectodermal pit formed on the ex-umbral surface of a medusa-person. C. Chun and, more recently, R. Woltereck [59], on the other hand, have shown that the ectodermal pit which gives rise to the pneumatophore represents an entocodon. Hence the cavity of the air-sack is equivalent to a sub-umbral cavity in which no manubrium is formed, and the pore or orifice of invagination would represent the margin of the umbrella. In the wall of the sack is a double layer of endoderm, the space between which is a continuation of the coelenteron. By coalescence of the endoderm-layers, the coelenteron may be reduced to vessels, usually eight in number, opening into a ring-sinus surrounding the pore. Thus the disposition of the endoderm-cavities is roughly comparable to the gastrovascular system of a medusa.
The difference between the theories of Haeckel and Chun is connected with a further divergence in the interpretation of the stem or axis of the cormus. Haeckel regards it as the equivalent of the manubrium, and as it is implanted on the blind end of the pneumatophore, such a view leads necessarily to the air-sack and gland being a development on the ex-umbral surface of the medusa-person. Chun and Woltereck, on the other hand, regard the stem as astolo proliferarising from the aboral pole, that is to say, from the ex-umbrella, similar to that which grows out from the ex-umbral surface of the embryo of the Narcomedusae and produces buds, a view which is certainly supported by the embryological evidence to be adduced shortly.
In the siphosome the following types of appendages occur:—
1.Siphonsor nutritive appendages, from which the order takes its name; never absent and usually present in great numbers (fig. 68, e). Each is a tube dilated at or towards the base and containing a mouth at its extremity, leading into a stomach placed in the dilatation already mentioned. The siphons have been compared to the manubrium of a medusa-individual, or to polyps, and hence are sometimes termed gastrozoids.
2.Palpons(fig. 68,g), present in some genera, especially in Physonectae; similar to the siphons but without a mouth, and purely tactile in function, hence sometimes termed dactylozoids. If a distal pore or aperture is present, it is excretory in function; such varieties have been termed “cystons” by Haeckel.
3.Tentacles(“Fangfäden”), always present, and implanted one at the base of each siphon (fig. 68,f). The tentacles of siphonophores may reach a great length and have a complex structure. They may bear accessory filaments ortentilla(f′), covered thickly with batteries of nematocysts, to which these organisms owe their great powers of offence and defence.
4.Palpacles(“Tastfäden”), occurring together with palpons, one implanted at the base of each palpon (fig. 68,h). Each palpacle is a tactile filament, very extensile, without accessory filaments or nematocysts.
5.Bracts(“hydrophyllia”), occur inCalycophoridaand somePhysophoridaas scale-like appendages protecting other parts (fig. 68,l). The mesogloea is greatly developed in them and they are often of very tough consistency. By Haeckel they are considered homologous with the umbrella of a medusa.
BL, Blastostyle.
C, Centradenia.
D, Palpon.
EC, Edge of colony prolonged beyond the pneumatophore.
G, Cavity of the large central siphon.
M, Medusoid gonophores.
PN, Primary central chamber, and PN′, concentric chamber of the pneumatophore, showing an opening to the exterior and a “trachea.”
S, Sail.
6.Gonostyles, appendages which produce by budding medusae or gonophores, like the blastostyles of a hydroid colony. In their most primitive form they are seen inVelellaas “gonosiphons,” which possess mouths like the ordinary sterile siphons and bud free medusae. In other forms they have no mouths. They may be branched, so-called “gonodendra,” and amongst them may occur special forms of palpons, “gonopalpons.” The gonostyles have been compared to the blastostyles of a hydroid colony, or to the manubrium of a medusa which produces free or sessile medusa-buds.
7.Gonophores, produced either on the gonostyles already mentioned or budded, as in hydrocorallines, from the coenosarc,i.e.the stem (fig. 68,i.). They show every transition between free medusae and sporosacs, as already described, for hydroid colonies. Thus inVelellafree medusae are produced, which have been described as an independent genus of medusae,Chrysomitra. In other types the medusae may be set free in a mature condition as the so-called “genital swimming bells,” comparable to theGlobiceps of Pennaria. The most usual condition, however, is that in which sessile medusoid gonophores or sporosacs are produced.
The various types of appendages described in the foregoing may be arranged in groups termedcormidia. In forms with a compact coenosarc such asVelella,Physalia, &c., the separate cormidia cannot be sharply distinguished, and such a condition is described technically as one with “scattered” cormidia. In forms in which, on the other hand, the coenosarc forms an elongated, tubular axis or stem, the appendages are arranged as regularly recurrent cormidia along it, and the cormidia are then said to be “ordinate.” In such cases the oldest cormidia, that is to say, those furthest from the nectosome, may become detached (like the segments or proglottides of a tape-worm) and swim off, each such detached cormidium then becoming a small free cormus which, in many cases, has been given an independent generic name. A cormidium may contain a single nutritive siphon (“monogastric”) or several siphons (“polygastric”):
The following are some of the forms of cormidia that occur:—
1. Theeudoxome(Calycophorida), consisting of a bract, siphon, tentacle and gonophore; when free it is known asEudoxia.
2. Theersaeome(Calycophorida), made up of the same appendages as the preceding type but with the addition of a nectocalyx; when free termedErsaea.
3. Therhodalomeof someRhodalidae, consisting of siphon, tentacle and one or more gonophores.
4. TheathoromeofPhysophora, &c., consisting of siphon, tentacle, one or more palpons with palpacles, and one or more gonophores.
5. ThecrystallomeofAnthemodes, &c., similar to the athorome but with the addition of a group of bracts.
a, Axis of the colony.
m, Nectocalyx.
c, Sub-umbral cavity of nectocalyx.
v, Radial canals of nectocalyx.
o, Orifice of nectocalyx.
t, Bract.
n, Siphon.
g, Gonophore.
i, Tentacle.
Embryology of the Siphonophora.—The fertilized ovum gives rise to a parenchymula, with solid endoderm, which is set free as a free-swimming planula larva, in the manner already described (seeHydrozoa). The planula has its two extremities dissimilar (Bipolaria-larva). The subsequent development is slightly different according as the future cormus is headed by a pneumatophore (Physophorida, Cystophorida) or by a nectocalyx (Calycophorida).
(i.) Physophorida, for exampleHalistemma(C. Chun,Hydrozoa[1]). The planula becomes elongated and broader towards one pole, at which a pit or invagination of the ectoderm arises. Next the pit closes up to form a vesicle with a pore, and so gives rise to the pneumatophore. From the broader portion of the planula an outgrowth arises which becomes the first tentacle of the cormus. The endoderm of the planula now acquires a cavity, and at the narrower pole a mouth is formed, giving rise to the primary siphon. Thus from the original planula three appendages are, as it were, budded off, while the planula itself mostly gives rise to coenosarc, just as in some hydroids the planula is converted chiefly into hydrorhiza.
(ii.) Calycophorida, for example,Muggiaea. The planula develops, on the whole, in a similar manner, but the ectodermal invagination arises, not at the pole of the planula, but on the side of its broader portion, and gives rise, not to a pneumatophore, but to a nectocalyx, the primary swimming bell orprotocodon(“Fallschirm”) which is later thrown off and replaced by secondary swimming bells,metacodons, budded from the coenosarc.
From a comparison of the two embryological types there can be no doubt on two points; first, that the pneumatophore and the protocodon are strictly homologous, and, therefore if the nectocalyx is comparable to the umbrella of a medusa, as seems obvious, the pneumatophore must be so too; secondly, that the coenosarcal axis arises from the ex-umbrella of the medusa and cannot be compared to a manubrium, but is strictly comparable to the “bud-spike” of a Narcomedusan.
Theories of Siphonophore Morphology.—The many theories that have been put forward as to the interpretation of the cormus and the various parts are set forth and discussed in the treatise of Y. Delage and E. Hérouard (Hydrozoa[4]) and more recently by R. Woltereck [59], and only a brief analysis can be given here.
a′, Pneumatocyst.
t, Palpons.
a, Axis of the colony.
m, Nectocalyx.
o, Orifice of nectocalyx.
n, Siphon.
g, Gonophore.
i, Tentacle.
In the first place the cormus has been regarded as a single individual and its appendages asorgans. This is the so-called “polyorgan” theory, especially connected with the name of Huxley; but it must be borne in mind that Huxley regarded all the forms produced, in any animal, between one egg-generation and the next, as constituting in the lump one single individual. Huxley, therefore, considered a hydroid colony, for example, as a single individual, and each separate polyp or medusa budded from it as having the value of an organ and not of an individual. Hence Huxley’s view is not so different from those held by other authors as it seems to be at first sight.
In more recent years Woltereck [59] has supported Huxley’s view of individuality, at the same time drawing a fine distinction between “individual” and “person.” The individual is the product of sexual reproduction; a person is an individual of lower rank, which may be produced asexually. A Siphonophore is regarded as a single individual composed of numerous zoids, budded from the primary zoid (siphon) produced from the planula. Any given zoid is a person-zoid if equivalent to the primary zoid, an organ-zoid if equivalent only to a part of it. Woltereck considers the siphonophores most nearly allied to the Narcomedusae, producing like the buds from an aboral stolon, the first bud being represented by the pneumatophore or protocodon, in different cases.
Contrasting, in the second place, with the polyorgan theory are the various “polyperson” theories which interpret the Siphonophore cormus as a colony composed of more or fewer individuals in organic union with one another. On this interpretation there is still room for considerable divergence of opinion as regards detail. To begin with, it is not necessary on the polyperson theory to regard each appendage as a distinct individual; it is still possible to compare appendages with parts of an individual which have become separated from one another by a process of “dislocation of organs.” Thus a bract may be regarded, with Haeckel, as a modified umbrella of a medusa, a siphon as its manubrium, and a tentacle as representing a medusan tentacle shifted in attachment from the margin to the sub-umbrella; or a siphon may be compared with a polyp, of which the single tentacle has become shifted so as to be attached to the coenosarc and so on. Some authors prefer, on the other hand, to regard every appendage as a separate individual, or at least as a portion of an individual, of which other portions have been lost or obliterated.
A further divergence of opinion arises from differences in the interpretation of the persons composing the colony. It is possible to regard the cormus (1) as a colony of medusa-persons, (2) as a colony of polyp-persons, (3) as composed partly of one, partly of the other. It is sufficient here to mention briefly the views put forward on this point by C. Chun and R. Haeckel.
Chun (Hydrozoa[1]) maintains the older views of Leuckart and Claus, according to which the cormus is to be compared to a floating hydroid colony. It may be regarded as derived from floating polyps similar toNemopsisorPelagohydra, which by budding produce a colony of polyps and also form medusa-buds. The polyp-individuals form the nutritive siphosome or trophosome. The medusa-buds are either fertile or sterile. If fertile they become free medusae or sessile gonophores. If sterile they remain attached and locomotor in function, forming the nectosome, the pneumatophore and swimming-bells.
Haeckel, on the other hand, is in accordance with Balfour in regarding a Siphonophore as a medusome, that is to say, as a colony composed of medusoid persons or organs entirely. Haeckel considers that the Siphonophores have two distinct ancestral lines of evolution:
1. In theDisconanthae,i.e.in such forms asVelella,Porpita, &c., the ancestor was an eight-rayed medusa (Disconula) which acquired a pneumatophore as an ectodermal pit on the ex-umbrella, and in which the organs (manubrium, tentacles, &c.) became secondarily multiplied, just as they do inGastroblastaas the result of incomplete fission. The nearest living allies of the ancestralDisconulaare to be sought in thePectyllidae.
p, Pneumatophore
n, Nectocalyx.
l, Aurophore.
lo, Orifice of the aurophore.
s, Siphon.
t, Tentacle.
2. In theSiphonanthae,i.e.in all other Siphonophores, the ancestral form was aSiphonula, a bilaterally symmetrical Anthomedusa with a single long tentacle (cf.Corymorpha), which became displaced from the margin to the sub-umbrella. TheSiphonulaproduced buds on the manubrium, as many Anthomedusae are known to do, and these by reduction or dislocation of parts gave rise to the various appendages of the colony. Thus the umbrella of theSiphonulabecame the protocodon, and its manubrium, the axis or stolon, which, by a process of dislocation of organs, escaped, as it were, from the sub-umbrella through a cleft and became secondarily attached to the ex-umbrella. It must be pointed out that, however probable Haeckel’s theory may be in other respects, there is not the slightest evidence for any such cleft in the umbrella having been present at any time, and that the embryological evidence, as already pointed out, is all against any homology between the stem and a manubrium, since the primary siphon does not become the stem, which arises from the ex-umbral side of the protocodon and is strictly comparable to a stolon.
Classification.—The Siphonophora may be divided, following Delage and Hérouard, into four sub-orders:
I.Chondrophorida(DisconectaeHaeckel,TracheophysaeChun). With an apical chambered pneumatophore, from which tracheal tubes may take origin (fig. 70); no nectocalyces or bracts; appendages all on the lower side of the pneumatophore arising from a compact coenosarc, and consisting of a centralprincipal siphon, surrounded by gonosiphons, and these again by tentacles.