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GASTROTRICHA

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Originally appearing in Volume V11, Page 527 of the 1911 Encyclopedia Britannica.
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GASTROTRICHA , a small See also:

group of fairly See also:uniform animals which live among Rotifers and See also:Protozoa at the bottom of ponds and marshes, hiding amongst the recesses of the See also:algae and sphagnum and other fresh-See also:water See also:plants and eating organic debris and See also:Infusoria. They are of See also:minute See also:size varying from one sixtieth to one-three-hundredth of an See also:inch, and they move by means of See also:long See also:cilia. Two ventral bands composed of See also:regular transverse rows of cilia are usually found. The See also:head bears some especially large cilia. The cuticle which covers the See also:body is here and there raised into overlapping scales which may be prolonged into bristles. An enlarged, frontal See also:scale may See also:cover the head, and a See also:row of scales separates the ventral ciliated areas from one sz6 surfaces of the See also:nerve-end cells nearer the See also:lens instead of with the opposite end. The significance of this arrangement is not known, but it is important to See also:note, as shown by V. Henson, S. J. Hickson and others, that in the bivalves Pecten and Spondylus, which also have eyes upon the See also:mantle quite distinct from typical cephalic eyes, there is the same relationship as in Oncidiidae of the optic nerve to the retinal cells. In both Oncidiidae and Pecten the pallial eyes have probably been See also:developed by the modification of tentacles, such as coexist in an unmodified See also:form with the eyes. The Oncidiidae are, according to K.

See also:

Semper, pursued as See also:food by the leaping See also:fish Periophthalmus, and the dorsal eyes are of especial value to them in aiding them to See also:escape from this enemy. Sub-See also:order I.—BASOMMATOPHORA. Pulmonata with an See also:external See also:shell. The head bears a single pair of contractile but not invaginable tentacles, at the See also:base of which are the eyes. Penis at some distance from the See also:female See also:aperture, except in Amphibola and Siphonaria. All have an osphradium, except the Auriculidae, which are terrestrial, and it is situated outside the pallial cavity in those forms in which water is not admitted into the See also:lung. There is a veliger See also:stage in development, but the velum is reduced. Fam. 1. Auriculidae. Terrestrial and usually littoral; genital duct monaulic, the penis being connected with the aperture by an open or closed groove; shell with a prominent See also:spire, the See also:internal partitions often absorbed and the aperture denticulated. See also:Auricula.

Cassidula. Alexia. See also:

Melampus. Carychium, terrestrial, See also:British. Scarabus. Leuconia, British. Blauneria. Pedipes. Fam. 2.—Otinidae. Shell with See also:short spire, and wide See also:oval aperture; tentacles short. Otina, British.

Camptonyx, terrestrial. Fam. 3.—Amphibolidae. Shell spirally coiled; head broad, without prominent tentacles; See also:

foot short, operculated; marine. Amphibola. Fam. 4.—Siphonariidae. Visceral See also:mass and shell conical; tentacles atrophied; head See also:expanded; genital apertures contiguous; marine animals, with an aquatic pallial cavity containing secondary branchial laminae. Siphonaria. Fam. 5.—Gadiniidae. Visceral mass and shell conical; head flattened; pallial cavity aquatic, but without a branchia ; genital apertures separated.

Gadinia. Fam. 6.—Chilinidae. Shell ovoid, with short spire, wide aperture and folded See also:

columella; inferior pallial See also:lobe thick; visceral commissure still See also:twisted. Chilina. Fam. 7.—Limnaeidae. Shell thin, dextral, with prominent spire and oval aperture; no inferior pallial lobe. Limnaea, British. Amphipeplea, British. Fam. 8.—Pompholygidae.

Shell dextral, hyperstrophic, See also:

animal sinistral. Pompholyx. Choanomphalus. Fam. 9.—Planorbida-e. Visceral mass and shell sinistral; inferior pallial lobe very prominent, and transformed into a branchia. Planorbis, British. Bulinus. Miratesta. See also:Fain. to. Ancylidae. Shell conical, not See also:spiral; inferior pallial lobe transformed into a branchia.

Ancylus, British. Latia. Grundlachia. Fain. ii.—Physidae. Visceral mass and shell sinistrally coiled; shell thin, with narrow aperture; no inferior pallial lobe. Physa, British. Aplexa, British. Sub-order 2.—STYLOMMATOPHORA. Pulmonata with two pairs of tentacles, except Janellulae and Vertigo; these tentacles are invaginable, and the eyes are See also:

borne on the summits of the posterior pair. Male and female genital apertures open into a See also:common See also:vestibule, except in Vaginulidae and Oncidiidae. Except in Oncidium, there is no longer a veliger stage in development. Tribe I.—HOLOGNATHA. See also:jaw See also:simple, without a See also:superior appendage.

Fam. 1.—Selenitidae. Radula with elongated and pointed See also:

teeth, like those of the Agnatha; a jaw See also:present. Plutonia. Trigonochlamys. Fam. 2.—Zonitidae. Shell external, smooth, heliciform or flattened; radula with pointed marginal teeth. Zonites, British. Ariophanta. Orpiella. Vitrina.

Helicarion. Fam. 3.—Limacidae. Shell internal. Limax, British. Parmacella. Urocyclus. Parmarion. Amalia. Agriolimax. Mesolimax. Monochrome.

Paralimax. Metalimax. Fam. 4.—Philomycidae. No shell; mantle covers the whole See also:

surface of the body; radula with squarish teeth. Philomycus. Pam. 5.—Ostracolethidae. Shell largely chitinous, not spiral, its calcareous See also:apex projecting through a small hole in the mantle. Ostracolethe. Fam. 6.—Arionidae.

Shell internal, or absent; mantle restricted to the anterior and See also:

middle See also:part of the body; radula with squarish teeth. Anion, British. Geomalacus. Ariolimax. Anadenus. Fam. 7.—Helicidae. Shell with See also:medium spire, external or partly covered by the mantle; genital aperture below the right posterior tentacle; genital apparatus generally provided with A. part-See also:sac and multifid vesicles. See also:Helix, British. Bulimus. Hemphillia. Berendtia.

Cochlostyla. Rhodea. Fam. 8.—Endodontidae. Shell external, spiral, generally ornamented with ribs; See also:

borders of aperture thin and not reflected; radula with square teeth; genital ducts without See also:accessory another, whilst two See also:series of alternating rows cover the back and See also:side. The body, otherwise circular in See also:section, is slightly flattened ventrally. The mouth is anterior and slightly ventral; it leads into a protrusible pharynx armed with recurved teeth that can be Bo everted. This leads to a See also:muscular Mr See also:oesophagus with a triradiate lumen, w4 which acts as a sucking See also:pump and ....h,a ends in a See also:funnel-See also:valve projecting LT into the See also:stomach. The last named is oval and formed of four rows of large cells; it is separated by a sphincter from the rectum, which opens posteriorly and dorsally. The nitrogenous excretory apparatus consists of a coiled See also:tube on each side of the stomach; internally the tubes end in large See also:flame-cells, and externally by small pores which See also:lie on the edges of the ventral row of scales. A cerebral ganglion rests on the oesophagus and supplies the cephalic cilia and hairs; it is continued some way back as two dorsal nerve trunks. The sense See also:organs are the hairs and bristles and in some See also:species eyes.

The muscles are simple and unstriated and for the most part run longitudinally. The two ovaries lie at the level of the juncture of the stomach and d.S' rectum. The eggs become very From Zeitschrift See also:

fir Wissen- large, sometimes See also:half the length of chalo zootogie, vol. x&x. p. 209, the See also:mother; they are laid amongst by permission of Wilhelm See also:Engel- maim. water weeds. The male reproductive Chaetonotus See also:maximus, See also:system is but little known, a small Ehrb., ventral side. (After gland lying between the ovaries has Zelinka.) been thought to be a testis, and if Bo, Bristles surrounding the mouth. it be, the Gastrotricha are herma- ds, Dorsal bristles. phrodite. hCi, Posterior lateral cilia. Zelinka classifies the group as fol- Ke, Cuticular See also:dome. lows : Mr, Oral cavity. Sub-order 1.—EUICHTHYDINA with a IT, Lateral sensory hairs. forked tail. P1, Cuticular plates. (i.) Fam. Ichthydidae, without Sa.

Dorsal bristle of the bristles. Genera: Ichthydium, Lepido- basal part. See also:

derma. Sch, Plates. (ii.) Fam. Chaetonotidae, with Se. Lateral bristles. bristles. Genera : Chaetonotus, Vb, Point of See also:union of cili- Chaetura. ated See also:tract. Sub-order 2.—APODINA, tail not vCi, Anterior group of cilia. forked. Genera: Dasydytes, Gossea, vS, Ventral bristles of the Stylochaela. basal part. The genus Aspidiophorus recently described by Voigt seems in some respects intermediate between Lepidoderma and Chaetonotus.

Zelinkia and Philosyrtis are two slightly aberrant forms described by Giard from certain diatomaceous sands. Altogether there must be some See also:

forty to fifty described species. The group is an isolated one and shows no clear See also:affinities with any of the See also:great phyla. Those that are usually dwelt on are treated with the Rotifers and See also:Nematoda and Turbellaria.

End of Article: GASTROTRICHA

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