Chapter 13

fig113Fig. 113.—Diglena catellina.(After Weber.)A, Male;B, the pairin copula;C, female,p, Penis;te, testis.

Fig. 113.—Diglena catellina.(After Weber.)A, Male;B, the pairin copula;C, female,p, Penis;te, testis.

The male organs consist of a testis (Fig. 113, A,te) with accessory glands, a large seminal vesicle, and a protrusible or projecting penis (p). InNotommataandDiglenatrue intromission at the cloaca (B) has been seen by many observers; but itappears equally certain that in many cases the male bores into the body-wall of the female at any point, and deposits the spermatozoa in the body-cavity, so that they must pass through the wall of the oviduct to effect fertilisation. Maupas finds that the process of fertilisation is ineffective except upon such newly-hatched females as would otherwise be the parents of small male eggs; that fertilisation is inoperative even for these at a later age when their eggs have begun to mature; and that it is wholly useless for those that lay ordinary summer eggs. The parent of male or winter eggs would thus be comparable to the queen bee, which if not fertilised produces drones. These sexual relations find a close parallel in the Ostracod and Phyllopod Crustacea, as well as in many plant-lice (Homoptera).

Development.—This has only been fully studied in the summer egg; inBrachionusby Salensky,[263]inMelicertaby Joliet[264]; inEosphora digitataand several other species by Tessin[265]; inCallidinaandMelicertaby Zelinka,[266]the last two observers having utilised modern methods of research.[267]We shall base our account on Zelinka's observations. As in the case of most "parthenogenetic" eggs, the ovarian egg begins by a very uneven division to form two cells: the minute "first polar body" which undergoes no further development; and the definitive egg, which by its repeated divisions gives rise to the tissues and organs.

Segmentation is very unequal, and recalls that of Molluscs in several respects. The first division gives rise to a smaller and a larger cell. Both of these divide again, the latter unequally, so that now there are three smaller cells and one large one; and after repeated divisions of the small cells and unequal divisions of the larger one, a stage is reached where there are a number of small cells and one large one, which sinks in and is overgrown by the small ones. Just prior to this the large cell undergoes equal divisions; its cells are the "hypoblast" cells (Fig. 114,hyp), and give rise to the gullet, stomach, and intestine, with their appendages, and the generative organs; while the smaller cells constitute the "epiblast" (ep), which gives rise to the body-wall and muscles, to the cement glands, nervous system, pharynx and mastax, and probably to the kidneys.

fig114Fig. 114.—Development ofCallidina. (After Zelinka.)A, Early stage showing involution of granular cells (g), to form the mastax or gizzard.B, Involution complete.C, Second involution of epiblast cells to form pharynx.D, The embryo bent on itself at ventral fold (vf).E, Showing ingrowth of epiblast to form brain (br):an, involution of epiblast to form cloaca;br, brain;ep, epiblast;fg, involution to form cement glands of foot;g, granular cells;gi, gizzard;hyp, hypoblast;m, mouth;o, ovary;sp, salivary glands;vf, limiting body from foot.

Fig. 114.—Development ofCallidina. (After Zelinka.)A, Early stage showing involution of granular cells (g), to form the mastax or gizzard.B, Involution complete.C, Second involution of epiblast cells to form pharynx.D, The embryo bent on itself at ventral fold (vf).E, Showing ingrowth of epiblast to form brain (br):an, involution of epiblast to form cloaca;br, brain;ep, epiblast;fg, involution to form cement glands of foot;g, granular cells;gi, gizzard;hyp, hypoblast;m, mouth;o, ovary;sp, salivary glands;vf, limiting body from foot.

Owing to the elongation of the body within the narrow space of the egg the hinder part is bent up on the ventral surface (D, E); and this part, narrower than the rest, forms the foot, the centre of which is at first occupied by a column of hypoblast. The cloaca is now formed by a dorsal ingrowth of epiblast (the "proctodaeum") at the junction of the foot and the body (an). The hypoblast in the body anterior to the cloacal ingrowth forms the digestive apparatus; the part immediately behind forms the reproductive organs (o); and the hindmost part apparently disappears. An ingrowth of epiblast at the extreme tip of the foot gives rise to the cement glands (fg). The muscles arise from the epiblast cells. The disc arises from the modification of epiblast cells lateral to and behind the mouth, enclosing a so-called "polar area"; it is completed by the transformation of cells on the ventral side of the mouth. The brain (br) is formed by the multiplication of epiblast cells; and in Bdelloida a ventral ingrowth below the mouth forms the sub-oesophageal ganglion. The ciliated cup inMelicertais formed as a ventral hollow, only later on united with the ciliated furrow of the wreath by the lateral grooves.[268]InMelicertathe two eyes are formed in the polar area. The young as hatcheddiffers from the adult in the greater simplicity of its ciliary wreath; and in the tubicolous forms the cupped end of the foot-gland is ciliated, and two eyes are present on the polar area, which later sink in, and often disappear more or less completely. It is stated that the young hatched from winter eggs do not pass through this larval state.

fig115Fig. 115.—Stephanoceros eichhornii.(After Cubitt.)A, Dorsal view of the upper part in its tube:al, lateral antenna;e, eye;em, developing embryo in uterus;g, gizzard;s.g, median salivary gland.B, Extremity of foot.C, Lateral view of base of disc: am, median antenna;of, oral funnel;s.g, median salivary gland;tf, in the crop, indicates the ciliated tube prolonging the funnel;tr, horseshoe-shaped trochus.

Fig. 115.—Stephanoceros eichhornii.(After Cubitt.)A, Dorsal view of the upper part in its tube:al, lateral antenna;e, eye;em, developing embryo in uterus;g, gizzard;s.g, median salivary gland.B, Extremity of foot.C, Lateral view of base of disc: am, median antenna;of, oral funnel;s.g, median salivary gland;tf, in the crop, indicates the ciliated tube prolonging the funnel;tr, horseshoe-shaped trochus.

Classification.[269]

Order I. Flosculariaceae.—Females mostly tubicolous, attached by a long contractile foot. Disc produced into a wide funnel-shaped contractile cup, produced into lobes with long setae (Floscularia) or coarse cilia (Stephanoceros), or entire (Apsilidae); an outer row of fine cilia rarely present; trochus a horseshoe, open behind. Oral funnel a slender tube hanging freely into a large pharyngeal crop; trophi uncinate projecting freely into the crop. Kidneys often united by an anterior cross-piece. Body-wall often containing a definite system of canals, filled with refractive granules, and serving by their contraction todilate the disc. Males (Fig. 107, 1) and larvae vermiform with a ciliated pedal cup, and a simple wreath, with two eyes on the disc.

Fam. 1. Flosculariidae:FlosculariaOken,StephanocerosE. (Fig. 115).Fam. 2. Apsilidae:ApsilusMetschnikoff (Fig. 112, A),AcyclusLeidy,AtrochusWierzejski (Fig. 112, C).

Fam. 1. Flosculariidae:FlosculariaOken,StephanocerosE. (Fig. 115).

Fam. 2. Apsilidae:ApsilusMetschnikoff (Fig. 112, A),AcyclusLeidy,AtrochusWierzejski (Fig. 112, C).

The family Flosculariidae contains some most exquisite forms;Stephanoceros, the "Crown Animalcule," being probably the most lovely of the Class, and many of the Floscules coming not far behind. The Apsilidae are mostly mud-dwellers.

Order II. Melicertaceae.—Females (except inTrochosphaera) attached or tubicolous; tube variable. Disc with a dorsal gap (exceptConochilus) often two-lobed or corolla-like; a ventral lip often separating off a ventral ciliated cup continuous by a pair of gutters with the ciliated groove; trochus of stronger cilia than the cingulum. Trophi malleoramate in a distinct mastax. Intestine much curved dorsally, cloaca long eversible (exceptTrochosphaera). Males and larvae as in Order I.

fig116Fig. 116.—Melicerta ringens.(After Joliet).A, Side view;B, dorsal view.al, Lateral antennae;ci, cingulum seen by transparency;g, gizzard;p, pellet in ciliated cup, about to be deposited on edge of tube;tr, trochus.

Fig. 116.—Melicerta ringens.(After Joliet).A, Side view;B, dorsal view.al, Lateral antennae;ci, cingulum seen by transparency;g, gizzard;p, pellet in ciliated cup, about to be deposited on edge of tube;tr, trochus.

Fam. 3. Melicertidae:MelicertaE. (Fig. 116),LimniasSchrank,CephalosiphonE.,OecistesE.,LacinulariaE.,MegalotrochaE.,ConochilusE.,OctotrochaThorpe.Fam. 4. Trochosphaeridae:TrochosphaeraSemper (Fig. 118, D).

Fam. 3. Melicertidae:MelicertaE. (Fig. 116),LimniasSchrank,CephalosiphonE.,OecistesE.,LacinulariaE.,MegalotrochaE.,ConochilusE.,OctotrochaThorpe.

Fam. 4. Trochosphaeridae:TrochosphaeraSemper (Fig. 118, D).

The Melicertidae embrace a large number of tubicolous forms, many of which are social. This habit is especially noticeable inLacinularia socialis, which forms a gelatinous incrustation easily seen by the naked eye; and inConochilus volvox, which forms free-swimming globular aggregates, the young attaching themselves when hatched to the centre of the ball, and the ball splitting up into two as soon as undue pressure is exerted at the periphery by overcrowding. In this genus the eyes are very conspicuous in the adult, as they are in the similar free-swimming aggregates ofLacinularia racemovata.

Trochosphaera(Fig. 118, D) is remarkable for its peculiarspherical shape, the absence of a foot, the limitation of the viscera to the lower hemisphere, and the dorsal position of the ovary. But a reference to the figure will show that the outgrowth of a foot in the quadrant between the mouth and anus and the flattening of the upper hemisphere would bring its organs on the whole into close correspondence with those of the rest of the Order. It is recorded from South China, the Philippines, and North-East Australia, and has only been seen by Semper, the founder of the genus, and by Thorpe, who saw the male of the first species, and described a second.[270]

Order III. Bdelloida.—Females creeping like a leech, as well as swimming (males unknown), susceptible of desiccation and revival ("anabiotic"). Body telescopic at both ends. Disc (except inAdineta) chiefly composed of two dorsal lobes like kettle-drums, wholly retractile; a dorsal proboscis or trunk-like prolongation of the body ends in a ciliated, sensory, and adhesive cup used in crawling, and overhung by a pair of membranous flaps. Trophi ramate; brain with a ventral ganglion, forming a complete ring. Eyes, two on the proboscis or brain, or absent. Bladder a mere dilatation of the rectum. Foot often possessing blind spurs, as well as two or three retractile perforated toes, or forming a terminal disc perforated by numerous pores of the cement glands, rarely ciliated.[271]

Fam. 5. Philodinidae:PhilodinaE. (Fig. 110),RotiferSchrank,ActinurusE.,CallidinaE. (Fig. 109),AdinetaH.

This group is remarkable for the great resisting powers of its members to drought and to heat and cold when dried, a fact which may explain the absence of males, though Janson records the occurrence of winter eggs in four species ofCallidinaand inAdineta vaga. The body is often strongly pigmented; red inPhilodina roseola,Callidina scarlatina, andC. russeola, yellow inP. citrina,Rotifer citrinus, andDiscopus synaptae. Most of the species are dust- or moss-dwellers; some, such asRotifer vulgaris, are equally common in organic débris in infusions, pools, and ditches.Discopusadheres to the skin of the HolothurianSynapta.

Order IV. Asplanchnaceae.—Females ovoid, footless except inAsplanchnopus. Disc often bearing a pair of antennae; circular, often prolonged at the margin into two rounded lobes, interrupteddorsally, depressed at the ventral side into a deep ventral funnel. Trophi incudate (virgate inAscomorpha), mastax enlarged dorsally into a wide crop; stomach large, blind. Kidneys large, with a "recurrent duct" and numerous tags; bladder large. Brain large, with a median eye, and frequently paired smaller eyes at the base of the marginal processes of the disc; anterior antennae paired, relatively far back on dorsal surface. Males (Fig. 107, 5) relatively large, frequently found.

Fam. 6. Asplanchnidae:AsplanchnaG.,AsplanchnopusDe Guerne, (?)Ascomorpha, Perty, (?)DinopsWestern.

Order V. Scirtopoda.—Females of conical shape, with the body prolonged into hollow limb-like expansions (see p.201) moved by strong muscles, and ending in branched setose fins like the limbs of Crustacea. Disc as in Bdelloids, but not retractile. Foot represented by two subventral toes, ciliated, inconstant or absent. Trophi malleoramate. Eyes two, latero-ventral, on the disc. Male (Fig. 107, 8) conical, with simple setae.

Fam. 7. Pedalionidae:PedalionH. (Fig. 117),[272]HexarthraSchmarda.[273]

Order VI. Ploima.—Free-swimming forms, more rarely parasites, often adherent by their trophi to a host. Disc variable, often bearing within the cingulum a number of lobes fringed with coarse compound cilia. Foot rarely absent, marked off by a sharp constriction. Mastax variable, rarely malleoramate, never incudate or uncinate. Intestine not blind. Males small.[274]

Sub-Order A. Illoricata.—Ploima with a soft flexible integument; disc variable; ciliated auricles sometimes present (Synchaetidae, Notommatidae); foot rarely absent; trophi usually malleate.

Fam. 8. Microcodonidae:MicrocodonE.,MicrocodidesBergendal.Fam. 9. Rhinopidae:RhinopsH.Fam. 10. Hydatinidae:HydatinaE. (Fig. 106),NotopsH.,HudsonellaZach.,CyrtoniaRouss.Fam. 11. Synchaetidae:SynchaetaE.Fam. 12. Notommatidae:NotommataE.,PleurotrochaE.,CopeusG.,ProalesG.,FurculariaG.,EosphoraG.,TriophthalmusE.,DiglenaE. (Fig. 113),DistemmaE.,TriphylusE.,TaphrocampaG.,AlbertiaDuj.,BalatroClap.Fam. 13. Drilophagidae:DrilophagusVejdovsky.Fam. 14. Triarthridae:TriarthraE.,PolyarthraE.,PteroessaG.,PedetesG.

Fam. 8. Microcodonidae:MicrocodonE.,MicrocodidesBergendal.

Fam. 9. Rhinopidae:RhinopsH.

Fam. 10. Hydatinidae:HydatinaE. (Fig. 106),NotopsH.,HudsonellaZach.,CyrtoniaRouss.

Fam. 11. Synchaetidae:SynchaetaE.

Fam. 12. Notommatidae:NotommataE.,PleurotrochaE.,CopeusG.,ProalesG.,FurculariaG.,EosphoraG.,TriophthalmusE.,DiglenaE. (Fig. 113),DistemmaE.,TriphylusE.,TaphrocampaG.,AlbertiaDuj.,BalatroClap.

Fam. 13. Drilophagidae:DrilophagusVejdovsky.

Fam. 14. Triarthridae:TriarthraE.,PolyarthraE.,PteroessaG.,PedetesG.

fig117Fig. 117.—Pedalion mirum, female. (After Hudson.)A, Ventral view;B, side view.a, Median antenna;al, antero-lateral limb;an, anus;ci, cingulum;dl, dorso-median limb;e, eye;f, ciliated pedal processes;l, lip;m, mouth;pl, postero-lateral limb;tr, trochus;vl, ventro-median limb.

Fig. 117.—Pedalion mirum, female. (After Hudson.)A, Ventral view;B, side view.a, Median antenna;al, antero-lateral limb;an, anus;ci, cingulum;dl, dorso-median limb;e, eye;f, ciliated pedal processes;l, lip;m, mouth;pl, postero-lateral limb;tr, trochus;vl, ventro-median limb.

To this group belongs the eyelessHydatina, a classical object of study, common in greenish pools, whose male was the first male Rotifer to be figured by Ehrenberg (1838), though he did not recognise its nature, and gave it the name ofEnteroploea hydatina.Rhinopshas the back of the corona curiously prolonged forwards into a sort of proboscis bearing two eyes. Some species ofNotommataandProalesare distinctly annulated; inTaphrocampathe segmentation is so marked as to give the appearance of mesenteric septa extending inwards from the body-wall to the intestine.Microcodonhas a wreath which is very peculiar in its extreme simplicity, with the mouth nearly central, and the eye lying just dorsal to the mouth. The Triarthridae, which resemble the Scirtopoda in having strong leaping spines fringed by fine bristles, should perhaps be placed in the next sub-Order.

Sub-Order B. Loricata.—Ploima with a firm elastic cuticleof definite form, persistent after death, continuous, or divided by thinner strips into plates or shields, which again may be areolated. The cuticle may also be shagreened or embossed in various ways.

Fam. 15. Rattulidae:RattulusE.,MastigocercaE.,CoelopusG.,Diurella(?) Eyfurth.Fam. 16. Dinocharididae:DinocharisE.,ScaridiumE.,StephanopsE.Fam. 17. Salpinidae:SalpinaE.,DiaschizaG.,PloesomaHerrick,DiplaxG.,DiploisG.Fam. 18. Euchlanididae:EuchlanisE.,DapidiaG.,ApodoidesJoseph.Fam. 19. Cathypnidae:CathypnaG.,DistylaEckstein,MonostylaE.Fam. 20. Coluridae:ColurusE.,MetopidiaE.,MonuraE.,MytiliaG.,CochleareG.,DispintheraG.Fam. 21. Pterodinidae:PterodinaE.,PompholyxG.Fam. 22. Brachionidae:BrachionusE.,NoteusE.,SchizocercaDaday.Fam. 23. Anuraeidae:AnuraeaE.,NotholcaG.,EretmiaG.

Fam. 15. Rattulidae:RattulusE.,MastigocercaE.,CoelopusG.,Diurella(?) Eyfurth.

Fam. 16. Dinocharididae:DinocharisE.,ScaridiumE.,StephanopsE.

Fam. 17. Salpinidae:SalpinaE.,DiaschizaG.,PloesomaHerrick,DiplaxG.,DiploisG.

Fam. 18. Euchlanididae:EuchlanisE.,DapidiaG.,ApodoidesJoseph.

Fam. 19. Cathypnidae:CathypnaG.,DistylaEckstein,MonostylaE.

Fam. 20. Coluridae:ColurusE.,MetopidiaE.,MonuraE.,MytiliaG.,CochleareG.,DispintheraG.

Fam. 21. Pterodinidae:PterodinaE.,PompholyxG.

Fam. 22. Brachionidae:BrachionusE.,NoteusE.,SchizocercaDaday.

Fam. 23. Anuraeidae:AnuraeaE.,NotholcaG.,EretmiaG.

The group includes a number of very minute forms, besides others conspicuous both for size and beauty. A soft dorsal flap above the head occurs inStephanops; also in Coluridae, a large family of minute species, where the flap is movable, and looks in profile like a hook overhanging the forehead. The genusPterodina, likePedalionandTriarthra, combines a Bdelloid disc with malleoramate trophi, while its exsertile wrinkled foot ends in a ciliated cup like that of a larval tubicolous species.

Brachionus, a large, often flat, transparent form, with a long wrinkled foot, is a very common genus, known to the earlier observers, and repeatedly figured by them.Pompholyxhas a sack-like lorica, no foot, and carries its immense egg suspended by an elastic thread from the cloaca. The Anuraeidae lack the foot, and often have great spines or bristles projecting from the lorica, which no doubt facilitate floating. They are abundant in the "plankton" or floating fauna of large lakes far from the shore. Many marine species belong to this family.

Order VII. Seisonaceae.—Marine Rotifers parasitic on the CrustaceanNebalia; males resembling the females. Body elongated, with a slender retractile neck, a much reduced disc, an elongated foot with a terminal perforated disc as inCallidina. Trophi virgate exsertile. Genito-urinary cloaca opening at the base of the neck in the male, at the hinder end of the body in the female. Intestine complete (Seison) or blind (Paraseison).[275]

Fam. 24. Seisonidae:SeisonGrube;ParaseisonPlate;SaccobdellaVan Beneden and Hesse.

Habits.—The habitat of Rotifers is well known to the student of pond life. Every dip from a greenish pool will give us a supply, if there be not an excessive contamination by manure; and such pools give us some of the largest and most beautiful forms, such asHydatinaandBrachionus, swimming about among the fibrous Algae and feeding on the organic débris among them. Almost any organic infusions freely exposed to the open air will yield Ploima shortly after the active putrefaction is completed. The finer water-weeds yield most of the beautiful tubicolous forms. A whole group of species and genera are quasi-pelagic in fresh and salt water, constituting a large proportion of the "plankton" or floating life near the surface; and some of these are found in deep water or in the depths of the lakes. Among them are the Asplanchnidae, Triarthridae, and Anuraeidae. A number of Loricates, such asNotholcaandEretmia, are armed with long spines, which doubtless render floating easier.

Among tubicolous formsConochilus volvoxandLacinularia racemovatahave this pelagic habit, forming floating globular or ovoid colonies, and two species ofFlosculariaalso float freely in their tubes.

The following forms occur in salt or brackish water,[276]those marked with an asterisk (*) also occurring in fresh water:—

Floscularia campanulata.* Melicerta tubicolaria.* Rotifer citrinus.* Discopus synaptae. Synchaeta baltica, S. monopus, S. apus, S. tremula,* S. longipes, S. tavina. Asplanchna girodi.* Asplanchnopus syringoides. Hexarthra polyptera. Notommata naias, N. reinhardti. Proales decipiens. Furcularia forficula,* F. gracilis, F. reinhardti, F. marina, F. neapolitana. Diglena catellina,* D. suilla, D. putrida. Pleurotrocha leptura. Distemma raptor, D. marinum, D. platyceps.* Bothriocerca longicauda. Polyarthra platyptera.* Triarthra longiseta.* Rattulus calyptus. Diurella marina, D. brevidactylus, D. brevis. Diaschiza fretalis. Euchlanis luna. Monostyla quadridentata, M. lunaris. Colurus amblytelus, C. uncinatus,* C. dactylotus, C. coelopinus, C. pedatus, C. rotundatus, C. truncatus, C. caudatus.* Mytilia tavina. Pterodina clypeata. Brachionus bakeri,* B. mülleri. Anuraea valga,* A. biremis,* A. aculeata,* A. tecta,* A. cochlearis.* Notholca striata,* N. scapha,* N. thalassia, N. spinifera, N. inermis, N. jugosa, N. rhomboidea. Seison grubei, S. annulatus. Paraseison asplanchnus, P. nudus, P. proboscideus, P. ciliatus. Discobdella nebaliae.

Thus about seventy species are recorded as marine.Synchaeta balticais truly pelagic, and contributes to the phosphorescence of the ocean.

Other forms again are parasitic.Proales werneckiiis found inVaucheria, a coarse, dark green, thread-like Alga found in fresh water; and the closely alliedP. parasitais not uncommon in the beautiful floating green spheres ofVolvox.[277]Albertia,Drilophagus, andBalatroare parasitic on or in fresh-water Oligochaetes; the curious Seisonaceae are parasitic onNebalia, a small Crustacean easily obtained in masses of whelk's eggs; the aberrant BdelloidDiscopusattaches itself to the surface of the HolothurianSynapta. Similarly among this last OrderCallidina parasiticaattaches itself to the limbs of the fresh-water CrustaceaGammarusandAsellus. These are rather commensals than true parasites. The species ofBrachionusoften attach themselves temporarily to the common water-fleaDaphnia.

Besides a few Ploima, the vast majority of the Bdelloids live in or among mosses and their roots. ManyCallidinainhabit cup-like hollows in the leaves of the scale mosses (Jungermanniaceae), especially of the genusFrullania. Almost all the members of this Order are susceptible of desiccation and revival; certain species, such asRotifer vulgaris,Philodina roseola,Adineta vaga, etc., can be readily obtained by moistening gutter dust. The mechanism of the process is as follows: when desiccation is gradual the animals close up their telescopic bodies and excrete gelatinous plugs at either end, which effectually seal them against further drying; if, however, they be dried on a slide without any débris, the process is too rapid for them to protect themselves, and they therefore die. This was dimly seen by others, and clearly demonstrated by H. Davis,[278]who records the following experiment:—The Rev. E. J. Holloway, having foundPhilodina roseolain gutters, placed strips of paper there in the rainy season, and succeeded in obtaining clean gatherings, taking dry groups of a hundred together, having a varnish-like covering all over; and being glued to one another, mostly in one plane, and to the paper, forming a pavement. In the dry condition they resist extremes of temperature; thus Zelinka foundCallidinarevive after an exposure of -20° C. (-4° F. or 36° of frost), and immersion in hot water at 70° C. (158° F.). They will also resist deprivation of air in a vacuum of an ordinary air-pump, but not the all but perfect exhaustion of the Sprengel pump.

A very curious fact in relation to this Class is that often when a new form is once described from a single locality, fresh and widely distant stations for it rapidly become known.[279]ThusPedalion mirum, first found at Clifton in 1872 by Hudson, was a few years after captured in a small pool above tide-marks on a rocky islet in Torres Straits. Since then it has been recorded from many different European stations, and a second closely allied species has been found in Finland. So a species of Ehrenberg's[280]was not seen again till within the last decade or so; but since then it has been independently found and described by six observers, who have given it as many distinct generic names. In the case ofPedalionit may well be that, as Hudson suggests, the species is of southern origin and has followed the flag, the winter egg being conveyed in dust by ships or travellers.

The above account of the habits gives the key to the collection of the various forms. The weed-loving species are collected with the weeds, and will keep with these in vessels if screened from direct sunlight and protected against dust. The free-swimming forms may be collected by sweeping with a net of fine gauze, with a bottle fixed in the bottom.

Except for their power of resisting desiccation, Rotifera are not very long-lived, and the males are especially short-lived; the most exact observations are those of Maupas onHydatina. He found that the greatest age of the unfertilised female was thirteen days, during which it could produce some fifty eggs; the fertilised female lives for seven or eight days, producing about sixteen eggs; while the male dies in two or three days.

The preservation of Rotifers has been recently reduced to a fine art by Rousselet, who uses a solution consisting of cocaine hydrochlorate, 1 gramme; water, 50 cc.; and methylated spirit, 12 cc. This will keep without deterioration. When in use itmust be diluted in the proportion of two volumes to three of water. This solution is added cautiously to the capsule in which the Rotifers lie, and they are watched till their ciliary motions slacken; when this happens a drop or two of osmic acid solution (½ to 1 per cent) is added; the Rotifers are then sucked up by a capillary pipette, and transferred to fresh water; and then into a solution of "Formaline" diluted to contain 2½ per cent of formic aldehyde. In this solution they are transferred to shallow cells, ground out of the centre of an ordinary glass slide, covered with thin glass, and sealed.[281]Other methods of preparing Rotifers for minute study will be found in the papers of Plate, Tessin, and Zelinka.

The zoological affinitiesof the Rotifers have long been a subject of keen interest. As early as 1851 Huxley[282]suggested that they represent a primitive form, preserved, with modifications, in the larva of Molluscs, Annelids and other worms, and Echinoderms. Similar views were later maintained by Lankester,[283]who termed the larva of Polychaets, etc., a "trochosphere," for which "trochophore" has been substituted in order to avoid confusion with the RotiferTrochosphaera; Balfour,[284]Hatschek,[285]Kleinenberg,[286]and others have developed these views. Serious difficulties, however, arise in the detailed comparison of Rotifers with this type; and the special students of this Class have found it practically impossible to agree in the identification of the various parts, a difficulty especially felt in the case of the Rotiferan genusTrochosphaera, though this is just the one which presents the closest superficial resemblance to the Trochophore larva. I have been induced to take a view of the structure of Rotifers that brings it into close relationship with the lower Platyhelminthes, and with the more primitive larva of the Nemertines termedPilidium(Fig. 60, p.113). This is hemispherical, ciliated all over, with the mouth in a ventral funnel lined by fine cilia; while the edge is fringed with two rows of strong cilia, separated by a finely ciliated groove, like those of the ciliary wreath of a Rotifer.

fig118Fig. 118.—Diagram explaining the possible relations of Rotifers.A,Pilidium;B, hypothetical Rotifer modified fromAsplanchnopus;C, a Ploimal Rotifer;D,Trochosphaera aequatorialis(modified from Semper, the extension of the ovary into the posterior ventral quadrant being omitted);E, Mollusc larva (Veliger);F, Trochophore larva of Annelid.a, Anus;ap, apical organ;at, median antenna (near which, inB, is a black spot, the brain);bl, bladder (receiving the ramified kidney inB,C, andD);br, brain;f, foot;fg, cement-glands, replacing apical organ;g, ovary;k, kidney;m, mouth;n, supra-oesophageal ganglion;nap, nerve of apical organ;nr, nerve-ring in section;pot, praeoral portion of trochus;s.g, shell-gland;s.n, sub-oesophageal ganglion;t, trochus or ciliary wreath;tt, posterior ciliated ring;v, velum, or expanded praeoral part of trochus.

Fig. 118.—Diagram explaining the possible relations of Rotifers.A,Pilidium;B, hypothetical Rotifer modified fromAsplanchnopus;C, a Ploimal Rotifer;D,Trochosphaera aequatorialis(modified from Semper, the extension of the ovary into the posterior ventral quadrant being omitted);E, Mollusc larva (Veliger);F, Trochophore larva of Annelid.a, Anus;ap, apical organ;at, median antenna (near which, inB, is a black spot, the brain);bl, bladder (receiving the ramified kidney inB,C, andD);br, brain;f, foot;fg, cement-glands, replacing apical organ;g, ovary;k, kidney;m, mouth;n, supra-oesophageal ganglion;nap, nerve of apical organ;nr, nerve-ring in section;pot, praeoral portion of trochus;s.g, shell-gland;s.n, sub-oesophageal ganglion;t, trochus or ciliary wreath;tt, posterior ciliated ring;v, velum, or expanded praeoral part of trochus.

The sides are produced on either side into lappets, which we do not take into account. A cup-shaped depression at the apical pole is lined by sense-cells, bearing long cilia which are probably sensory. A ring of nerve-cells passes within the ciliated rim of the hemisphere, and the stomach is a blind sac. If we compare this organism with a Rotifer, we find that the wreath corresponds in both, the funnel of the disc in such forms as Flosculariidae andMicrocodonleading to the mouth ofPilidium, while the gut is blind in Asplanchnidae and in some of the highly developed Seisonidae. The circular nerve-ring ofPilidiumis in many Rotifers only represented by its anterior part, the brain; though in Bdelloids a sub-oesophageal ganglion completes the ring. This leaves a difficulty with regard to the apical sense organ; but it is easy to understand that an organ of sensation should become an organ of fixation. In this case the foot with its glands would correspond to the sense organ of the Trochophore larva; and it retains its primitive ciliated character in the larvae and males of many Rotifera, and the adult female ofPterodinaandCallidina tetraodon. Embryology tellsus that the anus of Rotifers cannot be homologous with that of Annelids, etc., for it is formed outside the area of the blastopore: it is an independent formation, probably due to the coalescence of the originally blind intestine at its extremity with the earlier genito-urinary cloaca. On this view we must change the orientation of the Rotifer, and place it, like a Cuttlefish, mouth downwards: for "anterior and posterior" we must substituteoral(orbasal) andapical; for "dorsal" and "ventral" we must useanteriorandposterior; while "right" and "left" are unchanged. And this correctly expresses the actual space-relations in those Ploima likeRattulusthat swim with their disc in contact with the organic débris on which they feed, with the foot turned outwards and backwards. As these views are now published for the first time, I have thought it wiser to keep to the accepted relations in the general description, a course which has the advantage of avoiding difficulties in the study of the literature of the Class.

The supposed resemblance ofPedalionto the Crustacea is probably the result of convergence, not of consanguinity. The Polyzoa are a group of freely-budding organisms whose structure otherwise recalls in many respects that of the attached Rotifers; but a close investigation reveals so many differences in structure, orientation, and development, that we cannot regard the two groups as at all closely allied.

Thus the Rotifers may be regarded as a group apart, but probably representing an early offshoot from a free-swimming Platyhelminth, probably a Rhabdocoele; the modifications being the loss of the general ciliation of the surface, the arching of the back into an elongated vault, the conversion of the inner half of the pharynx into a gizzard, the change of position of the genital and urinary apertures to the antero-dorsal surface, and the opening of the intestine into the genito-urinary cloaca.

Gastrotricha.

This small and very homogeneous group consists of minute fresh-water organisms, closely resembling many Ciliate Infusoria in their movements, habit and habitat. They were first described in detail by Ehrenberg, and placed by him and Dujardin in the neighbourhood of Rotifers. In recent years A. C. Stokes[287]in America and C. Zelinka[288]in Germany have contributed, the former a careful description of a number of new species and their habits, the latter a complete monograph of everything that is known of the Order.

The Gastrotricha dwell among filamentous Algae and organic débris, and are of frequent occurrence with Protozoa and Rotifera of similar habit. The largest known measures only 400 µ (1⁄60in.) in length, and the smallest run as low as 74 µ (1⁄300in.).

fig119Fig. 119.—Gastrotricha. (From Zelinka.)A,Chaetonotus bogdanovii, side view (after Schimkewitsch);B,Gossea antenniger(after Gosse);C,Dasydetes goniathrix(after Gosse);D,Dasydetes saltitans(after Stokes);E,D. longisetosum(after Metschnikoff);F,Chaetonotus spinulosus(after Stokes);G,Chaetonotus schultzei(after Gosse and Bütschli). (Magnified.)B-F, × about 390;G, × about 125.

Fig. 119.—Gastrotricha. (From Zelinka.)A,Chaetonotus bogdanovii, side view (after Schimkewitsch);B,Gossea antenniger(after Gosse);C,Dasydetes goniathrix(after Gosse);D,Dasydetes saltitans(after Stokes);E,D. longisetosum(after Metschnikoff);F,Chaetonotus spinulosus(after Stokes);G,Chaetonotus schultzei(after Gosse and Bütschli). (Magnified.)B-F, × about 390;G, × about 125.

We shall follow Zelinka in his description of the common speciesChaetonotus larusas a type. The body is nearly circular in section, flattened a little on the ventral side. The apertures are the terminal mouth; the anus, nearly terminal and slightly dorsal; the two kidney openings, ventral, nearly half-way down the trunk; besides the pore of a cement-gland on either terminal process. The short ventral and post-anal portion of the trunk with its processes therefore corresponds to the foot of a Rotifer. The integument of the body is a thin nucleated hypoderm, notdistinctly divided into cells, covered by a chitinised cuticle; it bears cilia, sensory hairs, and peculiar scale-like processes, sometimes produced into long bristles.

The cilia are chiefly arranged in two ventral bands, each extending nearly the whole length of the body, and composed of a series of transverse rows of single cilia; along these bands the hypoderm is thickened and more richly nucleated. The sides of the head also bear numerous long cilia.

The scales are hollow processes of the cuticle overlapping from before backwards. A ventral row lies between the ciliary bands; two series of alternating dorsal rows lie on the back and sides of the animal, and in the hirsute species it is these that are produced backward into bristles. A single large scale, the "frontal shield," protects the head above and behind, but does not extend down to the ventral surface. On either side of the head is a pair of flattened oval areas, the "lateral fields." From between these on either side springs a tuft of motile sensory hairs. Two pairs of similar tufts arise dorsally on the front margin of the frontal shield, and a fourth pair spring from the ventral surface a little behind the mouth. These hairs are distinguished from ordinary cilia by their length, and their insertion on large nucleated cells receiving nerves; two pairs of similar hairs lie farther back on the dorsal surface, one in the front of the neck, one near the base of the pedal processes.

Themuscleslie some in the body-wall, and some traverse the body-cavity; only six pairs occur, simple, unstriated, and longitudinal. There are neither transverse nor circular muscles.

Thealimentary canalis very simple and nearly straight from mouth to anus; it may be divided into pharynx, gullet, stomach, and rectum. The mouth is circular, and looks forwards and a little downwards. From the mouth opens the pharynx, a short chitinous tube, capable of eversion by being pushed forwards by the gullet; it bears half-way down a circlet of curved hooks, which open out when it is everted; within these are tooth-like thickenings.

The oesophagus or gullet is thick and muscular, extending through the whole of the neck of the animal; its cavity, as well as the opening from the pharynx, is triradiate like a leech-bite, but can be dilated by the action of the muscular walls, inserted into a firm external cuticle; the internal wall is also cuticulised,not ciliated as in Rotifers. The hinder end of the gullet is produced into a short, wide, membranous funnel projecting freely into the midgut or stomach. The latter is elongated and oval, composed of four rows of hexagonal cells, with large nuclei. This is separated by a distinct constriction or sphincter from the short pear-shaped rectum, which opens by a minute anus on the back just in front of the pedal processes.

The food is chiefly organic débris; but Gastrotricha have been seen to attack large Infusoria by nibbling, and to swallow the protoplasm as it exudes from the wound in their prey.

Thenervous systemis chiefly composed of the large brain, a ganglion lying like a saddle above and on the sides of the gullet, and in direct continuity with the nerve-cells of the cephalic sense-hairs. A pair of dorsal nerve-trunks extend along the whole length of the gullet. The sense-hairs described with the general integument may be organs of external taste ("smell") or of touch. Eyes have been described in several species; and though Zelinka has failed to verify this, I have myself seen a pair of minute red eyes in the back of the head of an animal (probably aChaetonotus), whose hasty escape into a mass of débris prevented my determining its species.

Thekidneysare paired tubes lying at the sides of the front of the stomach, and sending a simple loop into the neck. Each tube is much convoluted, and ends at the one extremity in a long "flame-cell," like that of a Rotifer much drawn out, and at the other by a minute pore on the outer side of the ventral row of scales.

Reproductive Organs.—Only the female is with certainty known to occur; and the eggs, though recalling in their thick ornamented shell the fertilised winter eggs of Rotifers, are probably unfertilised and parthenogenetic like the summer eggs. The ovaries are two minute patches of cells lying at the junction of the stomach and rectum. The eggs, as they mature and enlarge, press against the side and back of the stomach, where they attain a length of one-third to one-half that of the mother. The extrusion of the egg has not been observed; but it is laid in the angles of weeds, the moulted shells of Entomostraca, etc., where its development may be studied. The sculpture of the shell serves to anchor it if laid among weeds. When hatched the head, trunk, and pedal processes are of the full adult size, all subsequent growth being limited to the neck.

The function of testis has been ascribed by Ludwig to a minute granular organ between the ovaries above the rectum; if this view be correct the Gastrotricha are hermaphrodite.

The movements of the Gastrotricha are very elegant, recalling those of the long-necked Ciliate Infusoria, likeAmphileptus, Lacrymaria, etc., with the characteristic exception that they always swim forwards; the grace of their movements being due to the bending of the head and neck on the body. Those which are provided with long motile bristles likeDasydetes, alternate their gliding with leaps, like the springing Rotifers.

The Gastrotricha are divided into two sub-Orders—Euichthydina, with two pedal appendages, containing the generaIchthydiumEhr.,LepidodermaZel.,ChaetonotusEhr., andChaeturaMetsch.; and theApodina, with no pedal appendages, comprisingDasydetesG. andGosseaZel.

Their geographical distribution, like that of most microscopic fresh-water organisms, is cosmopolitan. Few observers have enumerated the members of this group; of their extra-temperate occurrence we have only the single observations of Ehrenberg, Schmarda, and Voeltzkow for Nubia, Ceylon, and Madagascar respectively.

Of the thirty-two species described, twelve are recorded by A. C. Stokes from Maine and New Jersey only, besides five others that occur also in Europe. In Europe nineteen species are recorded, one of which,Ichthydium podura, has also been found in Nubia and Ceylon. One species,Chaetonotus tabulatusSchmarda, has been recorded by its author from Colombia (in South America). As of the nineteen European species only seven have been recorded as British, we may expect to find that careful study will well repay the student in these islands.

Theaffinitiesof this group are probably with the Turbellarians and the Nematodes; they differ from the former in the highly developed alimentary canal, and from the latter in the possession of the ciliated ventral bands and wreath. The general chitinisation of the skin, the primitive body-cavity, the character of the alimentary canal, the ventral opening of the renal canals far in front of the anus are characters shared by the Nematodes, many of which possess bristles like this group. But their affinity must be rather to some hypothetical ancestral group than to any living Nematodes, which are destituteof cilia. To the Rotifers the affinity, dwelt on by Zelinka, is less close.

Kinorhyncha.

This Class and Order comprises but one genus,Echinoderes(Fig. 120), founded in 1851 by Dujardin.[289]Reinhard's monograph[290]is the generally accepted authority on this subject, and contains a full bibliography, with diagnoses of the individual species, eighteen in number.

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