Chapter 24

fig216Fig. 216.—Specimens of the CoralHeteropsammia cochlea, withAspidosiphon heteropsammiarumorA. micheliniliving in a state of commensalism with them. (From Bouvier.)

Fig. 216.—Specimens of the CoralHeteropsammia cochlea, withAspidosiphon heteropsammiarumorA. micheliniliving in a state of commensalism with them. (From Bouvier.)

Dendrostomacontains but five species, which are all found within the tropics in the Pacific or in the West Atlantic. They are shallow-water forms, and some are found between tide-marks.

Phascolionis a smaller genus, containing but ten species, which may have been derived independently from different species ofPhascolosoma, and in this case the genus should be broken up. The members of this genus live in Mollusc shells, such asDentalium,Turritella,Buccinum,Chenopus(Aporrhais),Nassa,Strombus, and generally acquire the coiled shape of their host. They are usually attached to the shell by means of certainadhesive papillae found on their posterior end.Ph. strombifills its shell with mud, which must be kept together by some secretion of the animal. The body lies in a tube in this mud, and the introvert projects from the small round opening at the end of the tube, and explores the ground in every direction. They are found in all seas, but more especially in the colder waters.

fig217Fig. 217.—Cloeosiphon aspergillumQuatr. × ½.a, Introvert covered with spines and partially extended, but not sufficiently to show the head;b, calcareous plates surrounding the point of origin of the introvert.

Fig. 217.—Cloeosiphon aspergillumQuatr. × ½.a, Introvert covered with spines and partially extended, but not sufficiently to show the head;b, calcareous plates surrounding the point of origin of the introvert.

The genusAspidosiphonincludes nineteen species, which are, with few exceptions, exclusively confined to the Indian Ocean and neighbouring seas, including the Red Sea. The exceptions areA. armatusfrom the Norwegian coast, andA. müllerifrom the Mediterranean and Adriatic.A. truncatusis also stated to occur at Panama, the Bahamas, and at Mauritius. The remaining species almost all occur in the Malay Archipelago and neighbouring islands, and as was the case withPhymosoma, this part of the world seems to be the headquarters of the genus.A. müllerilives in the interstices of rocks and stones, and occasionally in disused Mollusc shells.

Two species ofAspidosiphonhave been described by Bouvier[484]living in a state of commensalism with two species of Madreporarian corals,Stephanoceris rousseauiandHeteropsammia cochlea, which live on and surrounding the shells of certain Molluscs at Aden (Fig. 216). Apparently the Gephyrean takes up its abode within its house at a tender age, and according to Bouvier, it provides for its increasing bulk by secreting a coiled calcareous tube, the outer surface of which affords space for the growth of the coral.

The genusCloeosiphon, theEchinosiphonof Sluiter, includes three species:C. aspergillum(Fig. 217),C. molle, andC. javanicum. The first named occurs at Mauritius, the Malay Archipelago, and neighbouring islands; the others areconfined to the last-named area, which thus again forms the headquarters of a genus.

Golfingia, described by Lankester from a single specimen, was dredged in St. Andrews Bay, at the depth of 10 fathoms.

Petalostomacomprises but one species,P. minutum, which is found in the English Channel.

Onchnesomacomprises two species,O. steenstrupiiandO. sarsii, both found off the coast of Norway at considerable depths between 200 and 300 fathoms.

Tylosomacomprises one species,T. lütkenii, also from the Norwegian coast. It is dredged from stony ground in 50 to 80 fathoms.

II. Order Priapuloidea.

Anatomy.—This Order consists of the two generaPriapulusandHalicryptus. Both are cylindrical animals with the mouth at one end and the anus at the other. The introvert is short, and is covered with rows of chitinous spines, which are continued to some extent over the body.

The skin is folded in a series of rings, and the body is usually somewhat swollen posteriorly.P. caudatusbears a curious caudal appendage, beset with a number of hollow lobes somewhat grape-like in appearance. This is situated ventral to the anus; its lumen is continuous with that of the body-cavity, but it can be separated from it by the action of a sphincter muscle. Two such appendages exist inP. bicaudatus.

There cannot be said to be any head in the Priapuloidea; they have no tentacles or tentacular fringe, no proboscis, and no distinct brain; simply a round aperture, the mouth, which is surrounded by a groove in the skin, at the bottom of which the circumoesophageal nerve-cord lies. The mouth leads into a very muscular pharynx lined with stout chitinous teeth; this passes into an intestine, which is as a rule straight, but inP. glandiferit has a single loop.

The Priapuloidea possess no vascular system and no brown tubes. Their skin has in the main the same structure as that of the Sipunculids, with spines, glandular bodies, and papillae with sensory hairs which resemble similar structures onPhymosoma varians. Retractor muscles arise from the longitudinal musclesof the skin, and are inserted into the pharynx; they are short and not constant in number.

The nervous system has retained throughout its primitive connexion with the epidermis. In almost all animals the nervous system is formed from the epiblast or outermost cellular layer of the embryo; it usually, however, breaks away from this and sinks into the body. Thus inSipunculusit lies within the body-cavity, and has retained its primitive connexion with the outer layers of the skin only in the region of the brain; but in the Priapulids the nervous system, which consists of a ring round the mouth and of a ventral cord, lies embedded in the skin, and the nerve cells are directly continuous with the cells of the epidermis. The nerve-ring lies at the base of a groove in the skin, which forms a kind of gutter round the mouth; the ventral nerve-cord is visible exteriorly as a light line which marks the ventral surface of the animal. In no place is the ring or cord differentiated in any way, and there cannot be said to be any brain or special sense-organs. Numerous nerves are given off from the ring to the pharynx and intestine, and from the cord to the body-wall.

fig218Fig. 218.—Priapulus caudatusLam. Nat. size.a, Mouth surrounded by spines.

Fig. 218.—Priapulus caudatusLam. Nat. size.a, Mouth surrounded by spines.

The sexes are distinct, but they differ from the other Gephyrea in the nature of their reproductive organs. In mature specimens the ovaries or testes are easily recognisable, lying to the right and left of the alimentary canal. The reproductive glands are continuous with ducts, which act as oviducts and vasa deferentia respectively. Both glands and ducts are attached to the body-wall by a mesentery.

The excretory function is performed in the Priapuloidea by the ducts of the generative organs. These are primarily connected with a number of branching canals of small size which project into the body-cavity. According to Schauinsland,[485]one or more pear-shaped cells are found at the end of each branch, and each iscontinued into a long cilium which hangs down into the lumen of the canal, and by its movement produces a flickering motion. Beyond the free end of the large cilium the canal is lined with ciliated cells. The remarkable resemblance this form of excretory organ presents to that of the Platyhelminthes (videp.25) and of certain Chaetopods is worthy of attention. In the young Priapuloidea the duct with its branching canals is not masked by the generative organs, but as the animals become mature, diverticula from the duct arise, and the cells covering these become modified into ova in the female, and into spermatozoa in the male. The presence of these follicles masks the excretory part of the gland. The ova and spermatozoa escape through the ciliated ducts which open to the exterior one on each side of the anus, and, contrary to what is the case with other Gephyrea, leave the body without having ever been in the body-cavity.

Nothing is known of the embryology of either member of this family, but both genera appear to be sexually mature from the end of May until October.

Classification.—The two genera which make up the Order Priapuloidea are characterised as follows:—

Priapulus.—The body is continued into one or two caudal appendages, beset with hollow papillae; these are ventral to the anus. The introvert forms ¼ to ⅓ of the total body-length; it is covered with spines in conspicuous longitudinal rows, the rest of the body being ringed. The retractor muscles are numerous, and are attached to the body-wall, some anteriorly and some posteriorly.

The genus includes the following five species:—

P. caudatusLam. (Fig. 218).Hab.Coasts of Greenland, Norway, Great Britain, the North Sea, and the Baltic.P. bicaudatusDan.Hab.North Sea and Arctic Ocean.P. glandiferEhlers.Hab.Coast of Greenland, North Sea.P. brevicaudatusEhlers.Hab.North Sea and Baltic, from ten fathoms.P. tuberculato-spinosusBaird.Hab.Falkland Islands.

P. caudatusLam. (Fig. 218).Hab.Coasts of Greenland, Norway, Great Britain, the North Sea, and the Baltic.

P. bicaudatusDan.Hab.North Sea and Arctic Ocean.

P. glandiferEhlers.Hab.Coast of Greenland, North Sea.

P. brevicaudatusEhlers.Hab.North Sea and Baltic, from ten fathoms.

P. tuberculato-spinosusBaird.Hab.Falkland Islands.

Halicryptus.—No caudal appendages. Introvert ⅒ to1⁄12of the total body length, with numerous spines arranged in close circles. Retractors numerous and all attached to the body-wall anteriorly.

H. spinulosusv. Sieb. (Fig. 219).Hab.North Sea, Arctic Ocean, and Baltic, in from two to fifty fathoms.

It will be noticed that with the exception ofP. tuberculato-spinosus, described by Baird from a single specimen, the whole family is confined to northern seas.

Habits.—The newly-captured specimens of bothP. caudatusandH. spinulosusare of a flesh colour, with a somewhat metallic sheen. According to Apel, the latter lived in an aquarium for more than five months, whilst the former died during the first month. When first introduced into the aquarium they immediately began to busy themselves in the mud or sand at its bottom, and very seldom showed themselves above it. They forced their way into the sand by alternately contracting and extending their introvert, and thePriapulusarranged itself so that a portion, often a very small one, of its caudal appendage was exposed to the water; this fact supports the view that the appendage is respiratory in function. When the animal buries itself deeply, the appendage does not relinquish its position at the surface of the sand, but stretches itself until it in some cases surpasses the length of the body. On the other hand,Halicryptus(Fig. 219), according to the same observer, lies with the anterior end, the mouth, projecting from the surface of the sand, or else it curves itself, so that both ends project into the water.

fig219Fig. 219.—Halicryptus spinulosusv. Sieb. × 6.a, Dark line indicating the position of the ventral nerve-cord;d, mouth surrounded by spines.

Fig. 219.—Halicryptus spinulosusv. Sieb. × 6.a, Dark line indicating the position of the ventral nerve-cord;d, mouth surrounded by spines.

Leckenby, who described specimens ofP. caudatuswhich were found by fishermen searching for worms for bait in the outer harbour at Scarborough at half tide, states that they live in sandy clay inU-shaped tubes, at a depth of about 9 inches, the tubes opening at each end on to the surface of the sand. The fishermen of this district call them "sea mushrooms."

Halicryptuscasts its cuticle in May and September; it becomes loose first at the hinder end, and the split between it and the skin grows forward until the animal lies free in a cuticular mantle. After some days this is split, and the animal frees itself from it; the cast-off cuticle includes for a shortdistance the lining of the mouth, the anus, and the two generative pores.

III. Order Echiuroidea.

Anatomy.—The most striking peculiarity of the Echiuroidea, as opposed to the other two families of the Gephyrea, is the presence of a solid dorsal outgrowth of a portion of the head, forming the proboscis. The nature of this proboscis is something quite different from that of the introvert of the Sipunculoidea; it would appear to correspond to an extension, in the members of the last-named Order, of that part of the head which is dorsal to the mouth and is covered by a peculiar pigment-epithelium, often in continuity with the brain. In its outgrowth this portion of the body has carried with it the nerve-ring and the vascular ring, which both surround the mouth. The proboscis is found in all the genera with the exception of the aberrant genusSaccosoma.

fig220Fig. 220.—A,Bonellia viridisRol., ♀;B,B. fuliginosa. Both nat. size.a, Grooved proboscis;b, mouth;c, ventral hooks;d, anus.

Fig. 220.—A,Bonellia viridisRol., ♀;B,B. fuliginosa. Both nat. size.a, Grooved proboscis;b, mouth;c, ventral hooks;d, anus.

The body of the femaleBonellia viridis, one of the best known species of Echiurids, is shaped like a small sausage, and is usually about 2 inches long. The proboscis arises from the anterior end, and is extremely extensible. At the distal end the proboscis splits into two short arms, which are often recurved; along the whole ventral surface runs a groove lined with cilia, which by the approximation of its edges can be converted into a tube. At the bottom of the proboscis the groove opens into the mouth.Echiurus;Thalassema, and the femaleHamingiahave short proboscides, which do not bifurcate but otherwise resemble those of the femaleBonellia.

fig221Fig. 221.—View of a femaleBonellia viridisRol., opened along the left side, × 2.a, Proboscis cut short;b, a bristle passed through the mouth into the pharynx;c, convoluted intestine;d, anal tufts or vesicles;e, ventral nerve-cord;f, ovary borne on ventral vessel running parallel withe;g, position of anus;h, points to position of external opening of nephridium;i, nephridium. This line is on a level with the internal funnel-shaped opening.

Fig. 221.—View of a femaleBonellia viridisRol., opened along the left side, × 2.a, Proboscis cut short;b, a bristle passed through the mouth into the pharynx;c, convoluted intestine;d, anal tufts or vesicles;e, ventral nerve-cord;f, ovary borne on ventral vessel running parallel withe;g, position of anus;h, points to position of external opening of nephridium;i, nephridium. This line is on a level with the internal funnel-shaped opening.

The green colour ofB. viridisis due to a special pigment, "Bonellein," which at one time was thought to be identical with chlorophyll. A similar green colour is found inHamingia arctica,Thalassema baronii, and the larvae of many forms.

A short distance behind the mouth, on the ventral surface, the femaleBonelliaand both sexes ofThalassemaandEchiurusbear two incurved stout chitinous hooks; these gave the nameGephyrea Armata to the above-mentioned genera. In addition to these,Echiurushas a row of chitinous bristles surrounding the posterior end of the body; the row is single inE. unicinctus, double inE. pallasii. These bristles are formed, like the hooks on the introvert of the Sipunculoidea, by epidermal cells; those ofB. minorand of the posterior rings inEchiurusare said to arise each from a single cell, just as the bristles do in Chaetopods.

The skin consists of very much the same layers as does that ofSipunculus; the cuticle is thin, the epidermis is modified into numerous glandular cells, papillae, and pits, from which the bristles arise. A third layer of oblique or circular fibres is usually found inside the longitudinal muscle-layer. The proboscis is solid, and contains much connective-tissue and numerous muscle-fibres running in all directions; the ventral groove is ciliated.

The alimentary canal in the Echiuroidea consists of a long thin-walled tube with numerous convolutions; it is not coiled as in Sipunculids, but the loops are irregularly arranged, and are supported by numerous fine muscular strands which run from the skin. There is a ciliated groove running along one side of the intestine, as in the Sipunculids. The anus is terminal. The most striking peculiarity of the alimentary canal of the Echiurids is the existence of a collateral intestine or "siphon." This is a narrow tube which arises from the main canal not very far from the mouth, and re-enters it again lower down. A similar structure occurs in some Echinids, and in the Capitelliformia (pp.272,305). Its function is not certainly known.

Another characteristic feature of the Echiurids is the presence of "anal vesicles," branching structures which unite into a common stem opening into the intestine close to the anus. The free end of each of the branches terminates in a ciliated funnel-shaped opening. The function of these structures may be excretory, or they may control the amount of fluid in the body-cavity.

A closed vascular system exists in Echiurids, consisting of a contractile dorsal vessel running along the dorsal surface of the anterior end of the alimentary canal, and continued along the axis of the proboscis. At the tip of the proboscis it bifurcates, and each branch descends along the edge until it reaches the base where, having encircled the oesophagus, the two unite, and are continued as the ventral vessel which runs along the dorsal surface of the nerve-cord, and eventually ends blindly. There is also a vessel whichpasses from the ventral vessel and encircles the intestine, opening into the posterior end of the dorsal vessel. InEchiurusthe same vessel encircles a stout muscle which runs from the base of one of the ventral bristles to the other. InThalassemaLankester states that the fluid within the vessels is colourless, and does not contain corpuscles similar to those in the body-cavity fluid.

The "brown tubes" or nephridia vary in number in the Echiurids. In the femaleBonelliathere is but one; inB. viridisthe right, inB. minorthe left usually persists. In shape, colour, contractility, and minute structure they closely resemble those ofSipunculus.Hamingiais said to have a pair of brown tubes;Echiurushas two pairs, exceptE. chilensis, which has three; their internal openings are produced into long coiled slits in some genera.Thalassema gigashas one pair;Th. neptuni,Th. baronii,Th. formosulum, andTh. exilii, two; whilstTh. vegrande,Th. moebii,Th. erythrogrammon,Th. caudex, andTh. sorbillanshave three pairs.

The nervous system consists of a ventral cord lying in the body-cavity, as in the Sipunculoidea, but attached to the skin, and of a circumoesophageal ring. With the growth of the proboscis this ring is drawn out, and the two branches run along the sides of the proboscis and unite at the tip. There is no specialisation of brain, nor are any special sense organs present, but the ventral cord gives off paired nerves at regular intervals, which, uniting dorsally, form rings in the skin in some and probably in all species.

The perivisceral fluid is of a dark brown colour inThalassema, containing numerous spherical corpuscles deeply impregnated, according to Lankester, with haemoglobin, and also containing granules of a brown pigment. Haemoglobin is also found in certain of the muscles and in part of the epithelial lining of the body-cavity. Lankester also describes the presence of haemoglobin in the corpuscles of the perivisceral fluid inHamingia.

The genital glands are, like those of the Sipunculoidea, formed by a special development of the cells lining the body-cavity. These cells are massed together along the wall of the ventral blood-vessel. InEchiurusand inThalassemathe cells break off and float in the body-cavity, developing into ova and spermatozoa. InBonelliaeach cell does not become an egg, but a mass of cells breaks off, one of which increases in size at the expense of theothers and forms the ovum. The mature sexual cells leave the body through the nephridia.

fig222Fig. 222.—An adult maleBonellia viridisRol. The original was 1.5 mm. long. The nervous system is not shown. (After Selenka.).a, Generative pore with spermatozoa coming out;b, anterior blind end of intestine attached to the parenchymatous tissue by muscular strands;c, green wandering cells containing chlorophyll;d, parenchymatous connective-tissue;e, epidermis;i, intestine;j, vas deferens;l, internal opening of vas deferens;m, the left anal vesicle;n, spermatozoa in the body-cavity.

Fig. 222.—An adult maleBonellia viridisRol. The original was 1.5 mm. long. The nervous system is not shown. (After Selenka.).a, Generative pore with spermatozoa coming out;b, anterior blind end of intestine attached to the parenchymatous tissue by muscular strands;c, green wandering cells containing chlorophyll;d, parenchymatous connective-tissue;e, epidermis;i, intestine;j, vas deferens;l, internal opening of vas deferens;m, the left anal vesicle;n, spermatozoa in the body-cavity.

BonelliaandHamingiapresent very interesting cases of sexual dimorphism. In both genera the female is an animal of considerable size with the normal structure of the Echiuroidea, but the male (Fig. 222) is a microscopic Planarian-like animal, which lives in the mouth and in the nephridia of the female. Both inBonellia[486]and inHamingiathe male is provided with a pair of hook-like ventral bristles; these are wanting in the femaleHamingia. The surface of the male is ciliated, and the skincontains circular and longitudinal muscle-fibres. The body-cavity is developed, but does not reach to either end of the body. The alimentary canal is closed, neither mouth nor anus existing; it is supported by regularly arranged dorso-ventral muscle strands. A nerve-ring and a ventral cord exist. There are also two rudimentary organs corresponding with the anal vesicles of the female, and a single nephridium which acts as a duct for the spermatozoa; the latter arise from modified cells lining the body-cavity.

In both sexes the larvae develop to a certain stage without showing any trace of sexual differentiation, but after this stage, the development of the male is to a certain extent arrested; in some respects, indeed, it undergoes retrogressive changes. At this time it is found clinging to the proboscis of the female, thence it makes its way to the mouth, where it undergoes its final change; and then creeping out, finds its way into the nephridium of the female, and spends the rest of its life there in a special recess cut off by a fold from the excretory part of this organ. InHamingia, however, Lankester, who first described the male, did not find any in the nephridia, but found five specimens, each1⁄12inch long, within the dilated pharynx of the female.

Development.—InBonelliaandHamingiait seems probable that the ova are fertilised in the nephridium of the female; in the other genera they are fertilised in the water after leaving the body of the mother.

InThalassemaandEchiurusthe growth of the embryo results in the formation of a typical Trochosphere larva, a type widely spread in the animal kingdom, being found in the Chaetopoda (Fig. 145, A), Polyzoa (p.510), and Mollusca. The large prae-oral lobe persists in the Echiuroidea as the proboscis; the mouth is ventral in position, with usually a ring of cilia encircling the body in front of and behind it; the anus is posterior and terminal. A pair of larval excretory organs are present, and a special nervous aggregation of cells at the apex of the prae-oral lobe is usually indicated by the presence of a bunch of long cilia.

The trunk of the Trochosphere is unsegmented, and in certain groups of animals it remains so, but in Chaetopods, and inEchiurusandThalassema, it elongates and becomes dividedup into a series of somites or segments. Of these there are fifteen inEchiurus, and apparently eleven inTh. mellita; in this stage the Gephyrean larvae have again so close a resemblance to the segmenting Chaetopod larvae as to be easily mistaken for them. The segmentation is shown in the following way: (i.) the middle layer of cells or mesoblast is typically segmented, and forms septa, which separate each segment from its neighbours; (ii.) the ventral nerve-cord arises as segmentally-arranged thickenings of the epiblast, which fuse together, but retain their segmented appearance for some time; (iii.) the skin shows the segmentation of the body both by the arrangement of the pigment and by bands of cilia. The latter are replaced in the adult by rows of spines, and on the fourteenth and fifteenth segments inEchiurus pallasiiby the two peri-anal circles of bristles. Each bristle, like those of Chaetopods, originates from a single cell.

The anal vesicles arise quite late in the development; when they have acquired their openings into the body-cavity, they seem to take in water. InThalassema, as described by Conn, this is accompanied by remarkable changes, amounting almost to a metamorphosis. The body increases in bulk fourfold, the cilia of the prae-oral ring disappear, and the animal now moves only by means of its proboscis; the pigment is absorbed, and all traces of segmentation disappear. A similar intaking of water is described by Spengel inBonellia. In this genus the larva, which is coloured bright green, and has two brown eye-spots, is not such a typical Trochosphere as is that ofEchiurusandThalassema.

fig223Fig. 223.—Echiurus pallasiiGuér. × ½.a, Mouth at the end of the grooved proboscis;b, ventral hooks;c, anus.

Fig. 223.—Echiurus pallasiiGuér. × ½.a, Mouth at the end of the grooved proboscis;b, ventral hooks;c, anus.

Nothing is known of the development ofHamingiaor ofSaccosoma.

Species of Echiuroidea.—Echiurus.Proboscis not bifurcated at the end. Two ventral hooks and a single or double peri-anal ring of bristles. The body is to a varying extent markedby rings bearing spines. Two or three (E. chilensis) pairs of nephridia, their external orifice often lengthened and spirally coiled. Both sexes alike.

Greef[487]mentions six species ofEchiurus, viz.E. pallasii,E. forcipatus,E. sitchaensis,E. chilensis,E. carabaicus, andE. chrysacanthophorus; to these must be addedE. unicinctus. It seems probable thatE. forcipatusof Reinhardt is identical withE. pallasii, although bigger, whilstE. sitchaensis,E. carabaicus, andE. chrysacanthophorusare inadequately described. The distribution of the remaining three species is as follows:—

E. pallasiiGuérin (Fig. 223). North Sea, Atlantic, English Channel.E. unicinctusDrasche. Japan.E. chilensisMax Müller. Chili.

E. pallasiiGuérin (Fig. 223). North Sea, Atlantic, English Channel.

E. unicinctusDrasche. Japan.

E. chilensisMax Müller. Chili.

Thalassema.—Proboscis rather pointed at the end, not bifurcated. No peri-anal bristles, but two ventral hook-like bristles placed anteriorly. One to three or four pairs of nephridia. The sexes resemble each other.

Greef mentions eight species ofThalassemaand Rietsch thirteen; three of these, however,Th. grohmanni,Th. lessonii, andTh. pelzelnii, were not seen by either author, and their description is taken from Diesing. There is some reason for thinking that the two first-named species are identical withTh. neptuni. Conn has established a new species for the specimens whose embryology he worked out at Beaufort, Virginia, and Selenka described a new species from the Challenger material.

With the exception of the three doubtful species mentioned above, the list of species ofThalassemais as follows:—

Th. neptuniGaertner (Fig. 224). English Channel (Devonshire coast), Concarneau, Mediterranean (Gulf of Marseilles), Irish coast (Dungarvan).Th. gigasMax Müller. Trieste.Th. vegrandeLampert. Philippine Islands.Th. baroniiGreef. Canary Islands (Lanzarote).Th. formosulumLampert. Shanghai and Philippine Islands.Th. exiliiFr. Müller. Brazil (Desterro).Th. moebiiGreef. Mauritius.Th. erythrogrammonMax Müller. Red Sea and East Indies (Billiton).Th. caudexLampert. Red Sea and Indian Ocean.Th. sorbillansLampert. Philippine Islands.Th. mellitaConn. West Atlantic (Beaufort).Th. faexSelenka. North of the Faroe Islands.

Th. neptuniGaertner (Fig. 224). English Channel (Devonshire coast), Concarneau, Mediterranean (Gulf of Marseilles), Irish coast (Dungarvan).

Th. gigasMax Müller. Trieste.

Th. vegrandeLampert. Philippine Islands.

Th. baroniiGreef. Canary Islands (Lanzarote).

Th. formosulumLampert. Shanghai and Philippine Islands.

Th. exiliiFr. Müller. Brazil (Desterro).

Th. moebiiGreef. Mauritius.

Th. erythrogrammonMax Müller. Red Sea and East Indies (Billiton).

Th. caudexLampert. Red Sea and Indian Ocean.

Th. sorbillansLampert. Philippine Islands.

Th. mellitaConn. West Atlantic (Beaufort).

Th. faexSelenka. North of the Faroe Islands.

Bonellia.—Proboscis very extensible and bifurcated at the end. The body and proboscis are coloured a bright green. Two ventral hook-like bristles, but no peri-anal ring. A single nephridium. The above applies to the female; the males are degenerate, and live in the nephridium or pharynx of the female.

Three (or four?) species of this genus are known.

B. viridisRolando (Fig. 220). Mediterranean, Adriatic, North Sea (Bergen).B. minorMarion. Mediterranean (Gulf of Marseilles, Naples).B. suhmiiSelenka. Off Nova Scotia. Male not known.B. fuliginosaRolando? (Fig. 220). Mediterranean (Naples).

B. viridisRolando (Fig. 220). Mediterranean, Adriatic, North Sea (Bergen).

B. minorMarion. Mediterranean (Gulf of Marseilles, Naples).

B. suhmiiSelenka. Off Nova Scotia. Male not known.

B. fuliginosaRolando? (Fig. 220). Mediterranean (Naples).

Hamingia.—Proboscis not bifurcated, about as long as body. No ventral hook-like bristles. One or two nephridia, which open at the apex of one or two well-marked papillae. The above applies to the female; as in the genusBonellia, the male is minute and parasitic. It has two well-marked hook-like bristles situated behind the genital pore.

This genus was first described by Koren and Danielssen asH. arctica. Two specimens were afterwards described by Horst asH. glacialis. Later Lankester described two other specimens; he was the first to find the male in the pharynx of the female. He is of the opinion that all three descriptions apply to the same species, and for this the original nameH. arcticamust be retained.

Hamingia arcticaK. and D. Two hundred miles north of North Cape and in the Hardanger Fjord.

Saccosoma.—No proboscis. The body is flask-shaped. The mouth and anus are terminal. The ovary is anterior, and there is only one nephridium. No bristles.

Our knowledge of this remarkable Gephyrean is very incomplete, but such as it is, it is due to the careful investigations of Koren and Danielssen, who had only a single specimen at their disposition.

Saccosoma vitreumK. and D. North of the Faroe Islands.

Habits of the Echiuroidea.—As a rule the members of this group conceal their bodies in clefts and fissures of rocks and stones, keeping up communication with the outer world by means of their proboscis. Rietsch[488]describes a specimen ofBonellia minor, which he placed in an aquarium, exploring with itsproboscis the nature of the bottom; when the animal had found a convenient crevice, it fixed its proboscis in it by means of the bifurcated end, and by its contraction drew the body up, and entered the hole, proboscis first. It then turned round, and during this operation doubtless the ventral hooks came into play; and then stretching out its proboscis, it began to explore the neighbourhood. The proboscis is evidently very sensitive, and in addition to being a locomotor organ, it is also used for the prehension of food. If cut off near the mouth, the animal does not long survive, but if a considerable portion is left the scar heals, and the lost part is probably regenerated. In captivity the animals frequently change their place of residence.

Eisig some years ago described the great extensibility of the proboscis ofB. viridiswhen confined in the tanks of the Zoological Station at Naples. When contracted the proboscis was but a few inches long, but at times it was extended till it reached the length of 1½ metre, shining through the water as a transparent green thread. The body of theBonelliawas hidden under stones, but the proboscis could be seen seizing between its two ends the bodies of certain Ascidians which covered the inside of the tank, tearing them off the walls, and conveying them to the mouth along its grooved ventral surface.

The food of the Echiuroidea consists of organic matter, in the main of animal nature, but the group differs from the Sipunculoidea in not eating sand.

Rietsch describesThalassema neptunias being more active in its movements and less sedentary thanB. minor. The proboscis is still the chief organ of locomotion, but the trunk plays a greater part in the movements of the animal than it does in the last-named species.Th. neptuniis found in cavities of stones or in the chambers worn out by the MolluscGastrochaena; when withdrawn from its house the body is found to be covered by a thick layer of tenacious viscid mucus.

fig224Fig. 224.—Thalassema neptuniGaert. × 2.A, The animal lying on its ventral surface.B, Ventral view of the anterior end, showing the grooved proboscis ending behind in the mouth, and the ventral hooks.

Fig. 224.—Thalassema neptuniGaert. × 2.A, The animal lying on its ventral surface.B, Ventral view of the anterior end, showing the grooved proboscis ending behind in the mouth, and the ventral hooks.

Th. mellitawas so named by Conn because it is found sheltering in the EchinidMellita. "It enters the shell at the oral opening while yet very small, but once within its house it grows to its adult size, and is obliged therefore to remain during the rest of its life a prisoner." Each shell thus inhabited acquires a reddish brown horse-shoe-shaped marking, which affords a conspicuous signal that the shell contains aThalassema.

Thalassemais seldom found living in sand, andBonellianever, butEchiurusis almost always found inU-shaped tubes or passages in the sand, which it digs out for itself by the rapid contractions of its body-wall aided by its bristles. It, like the other two genera named above, does not long remain in the same hole, but frequently changes its home. As a rule theEchiurussits near the mouth of its tube, which is often a foot or even two in depth, and sends out its proboscis in every direction; at the least sign of disturbance it withdraws into the deeper recesses. The walls of the tube are kept from falling in by a layer of mucus, which makes a smooth lining to the passage. The peri-anal bristles, which can be withdrawn or protruded at will, enable the animal to fix itself at any level in the tube.

The Echiuroidea are sometimes used by fishermen as bait. InEchiurus pallasiiGreef found three parasites, all of them new species. One, a Gregarine, he namedConorhynchus gibbosus; the others were Platyhelminthes, and were named by himDistomum echiuriandNemertoscolex parasiticusrespectively.

IV. Order Epithetosomatoidea.

This Order includes the single FamilyEpithetosomatidae, which was established by Koren and Danielssen to contain the remarkable Gephyrean they described in 1881 under the nameEpithetosoma norvegicum(Fig. 225).

Unfortunately only two specimens were at their disposition, and these were badly preserved, so that many details of their structure could not be made out. The animals are of an olive-green colour, and consist of a trunk about 12 mm. long, and of a proboscis 30 mm. in length; the latter differs essentially from the proboscis of the Echiuroidea inasmuch as it is hollow, and seems to be a whip-like tubular extension of the skin. Its lumen opens into the body-cavity. Ventral to the baseof the proboscis is the mouth; the intestine is straight, and terminates in the anus, which is posterior. The nervous system lies between the circular and the longitudinal muscles of the body-wall, and contains a tube, the nature of which is obscure. No vascular system is known. The ovary is attached to a mesentery ventral to the anterior part of the alimentary canal, and there is a single nephridium. No anal vesicles exist.

The most remarkable feature of the genus is a series of pore-like openings, which are stated to lead from the outside into the body-cavity (Fig. 225,a). These are arranged four on each side, at the bottom of two slit-like depressions in the skin, which lie one on each side of the base of the proboscis, slightly dorsal to it.

These remarkable structures are without parallel amongst the Gephyrea, and, together with the peculiar character of the proboscis, justify the Norwegian naturalists in adding a new family to the group.

fig225Fig. 225.—A,Epithetosoma norvegicumK. and D., magnified.a,a, Right and left slits leading to the pores;b, mouth;c, proboscis:B, the same animal opened dorsally;a, pores;b, oesophagus;c, proboscis;d, brown tube. (After Danielssen and Koren.)

Fig. 225.—A,Epithetosoma norvegicumK. and D., magnified.a,a, Right and left slits leading to the pores;b, mouth;c, proboscis:B, the same animal opened dorsally;a, pores;b, oesophagus;c, proboscis;d, brown tube. (After Danielssen and Koren.)

Affinities of the Gephyrea.

Before considering to what other groups of animals the Gephyrea may be allied, it is advisable to discuss the relationship of the four Orders which compose the group.

Quatrefages, in the year 1865, divided the Gephyrea into I.Gephyrea Armata, with which he included the Echiuroidea andSternaspis,[489]and II.Gephyrea Inermiaor Sipunculoidea. The Gephyrea Inermia, sometimes called the Achaeta, have been extended to include the Order Priapuloidea, and opposed to the smaller sub-group the Gephyrea Armata or Chaetifera. In my opinion, however, these names now are no longer in accordance with our knowledge of the structure of theanimals they attempt to describe, and they should be given up. Both names had reference to the presence or absence of the two hook-like bristles described on the ventral surface of some of the Echiuroidea, but of the five genera of this family, two,SaccosomaandHamingia(the latter in the female or normal form), are without these bristles, and can therefore be described neither asArmatanor asChaetifera. On the other hand, hook-like chitinous bristles of somewhat the same nature, though smaller in size and varying in position, are very common on the introvert of Sipunculoidea and on the body of the Priapuloidea.

Again, the association of the two last-named Orders in one sub-group is, to my mind, an error. The Priapuloidea have little in common with the Sipunculoidea; almost the only real point of resemblance is the power of protruding the anterior part of the alimentary canal, and withdrawing it by the aid of retractor muscles. But in the Priapuloidea this power exists to a very small extent, and it is a power shared by very many animals besides the Gephyrea. The terminal anus of the former is a feature shared by the Echiuroidea and byEpithetosoma, but these have little else in common with the Priapuloidea. On the other hand, the entire absence of any head appendages, such as the proboscis of the Echiuroidea and the tentacles or tentacular membrane of the Sipunculoidea, the absence of a vascular system, of nephridia or anal vesicles, taken together with the straight intestine which occurs elsewhere only inEpithetosoma, the persistent connexion of the nervous system with the epidermis, the unique character of their excretory system and of the reproductive organs, are all features in which the Priapuloidea differ from the more normal members of the other three Orders. These constitute a list of peculiarities which are at least as important, and probably even more important, than those which characterise the Sipunculoidea and the Echiuroidea. Thus the Priapuloidea should, I think, be regarded as a distinct Order, which occupies a very isolated position in the group.

Until we know something about the development ofHalicryptusand ofPriapulus, it will be difficult to say whether the Order is more nearly allied to one or the other of the two great Orders of Gephyrea, whether it is very primitive or very specialised. The connexion of the entire nervous system with the epidermis and the absence of a vascular system are bothrather primitive features, and so is the Platyhelminthine character of the excretory organs. With regard to the vascular system, however, it should be pointed out that it arises very late in the larva of those Gephyrea whose development is known, and that it does not seem to correspond with the vascular system of other animals; it has no fine vessels or capillaries connected with it, and apparently does not act so much as the channel of the circulatory medium, but more as a mechanism for the expansion of the head appendages, the tentacles in the Sipunculoidea and the proboscis in the Echiuroidea; moreover, it is absent in some genera of the former, such asOnchnesoma,Tylosoma, andPetalostoma, where there are no tentacles.

The conclusion of the whole matter seems to be that the Priapuloidea are an isolated Order retaining many primitive features, and having no closer affinities to the Sipunculoidea than to the Echiuroidea.

Hatschek came to the conclusion, from his work on the development ofEchiurus, that the Echiuroidea are true "Annelids," and from the presence and mode of formation of the bristles, that they are related to the Chaetopods. In this view he is confirmed by Conn, who worked out the development ofThalassema. This relationship is further confirmed by the discovery of Sluiter's thatSternaspis, the genus of Chaetopods which in other respects most nearly resembles the Gephyrea, has in one of its species (S. spinosa) a well-marked bifid proboscis, which, like that of the Echiuroidea, is thrown off at the least disturbance. Thus it seems fairly well established that the Echiuroidea are closely connected with the Chaetopoda, for although the only traces of segmentation they retain in the adult are the serially-repeated nephridia ofThalassemaandEchiurus pallasii, and the two rows of peri-anal bristles in the latter, and possibly the circular nerves given off from the ventral cord, yet the larva is fully segmented, and in other respects is almost typically Chaetopodan.

The relationship of the Sipunculoidea to the Echiuroidea is a more doubtful point. Hatschek is inclined to separate them, and in this he is again supported by Conn. Embryology unfortunately does not help us much. The early stages and larvae ofSipunculus nudusand ofPhascolosoma elongatumhave been investigated by Hatschek and by Selenka respectively. In neither genus is there any trace of segmentation or of Annelidfeatures, with the possible exception of the bristles on the larvalPhascolosoma. On the other hand, it must be remembered that the development ofSipunculusis remarkably abbreviated, and that such stages may have dropped out, the larvae hardly differing more from the Trochosphere ofEchiurusandThalassemathan does that ofBonellia, an undoubted Echiurid. Still the facts that there is never a head-kidney present, that there is no trace of segmentation, and that at no stage is the anus terminal, must have a certain weight.

If we leave out of account the larval history, which, although pointing to a difference in the nature of the two families, is by no means decisive, and consider the adult structures, we find very considerable evidences of affinity. Taking firstly the main points of difference, we find these to be (i.) the nature of the cephalic appendages, either a proboscis or some modification of tentacles; (ii.) the position of the anus; (iii.) the presence of anal vesicles; (iv.) the number of the nephridia, never more than one pair in Sipunculids; and (v.) the difference in origin of the chaetae. Of these most undoubtedly the first is the most important. The Echiuroidea have retained the prae-oral lobe of the larva in the form of a solid outgrowth of the body, which outgrowth has carried with it the nerve-ring and vascular ring which surround the mouth. This has been lost in the Sipunculoidea, but is, I think, represented by a modified patch of epidermis which lies dorsal to the mouth and just above the brain. A solid extension of the skin in this region, which involved the nervous and vascular systems, would bring about the same relation of parts as is found in the Echiuroidea. The tentacular membrane or tentacles of the Sipunculoidea have such a variety of form and arrangement, whilst all subserving the same end, that I am inclined to believe that they have originated within the limits of the family.

The position of the anus in the Sipunculoidea is one common to very many animals which live embedded in sand or in tubular holes; it is probably not primitive, as in the larva ofSipunculusit is near the posterior end, and becomes more dorsal as the larva elongates.

The anal vesicles of the Echiuroidea probably have no representative in the Sipunculoidea. In appearance and position they are very like the little tufts which are found on the rectum ofSipunculus, but since these open neither into the body-cavity nor into the alimentary canal, it is hardly fair to compare them.

The resemblances between the Orders seem to me, on the whole, to outweigh the differences. The general structure of the skin, the coiled alimentary canal, with its ciliated groove, supported by strands of muscles, the vascular system which gives off no capillaries, the structure of the brown tubes, the existence of chitinous hooks or bristles, the nervous system with its single unsegmented ventral cord, the formation of the generative organs, all point to a sufficiently close resemblance to justify us in classing the two Orders together. In addition to these there are considerable histological resemblances which cannot be discussed here, but which have a certain weight.

To sum up, it seems probable that the Echiuroidea are derived from the Chaetopoda, and that their nearest ally in this group isSternaspis; and that the Sipunculoidea are allied to the Echiuroidea, but have further departed from the Annelid stock, and have lost even those traces of affinity with the parent group which have been preserved in the development ofEchiurusandThalassema.

So little is known ofEpithetosomathat it is difficult to discuss its affinities. The presence of the hollow proboscis and the pores leading into the body-cavity undoubtedly justify its being placed in a separate Order, but beyond the presence of a terminal anus, in which it resembles the Echiuroidea, there is nothing in its structure which connects it more nearly with one than with the other of the three larger Orders of Gephyrea.

List of Gephyrea found in the British Area as defined by Canon Norman.

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