GASTROTRICHA  , a small

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

Semper, pursued as food by the leaping fish Periophthalmus, and the dorsal eyes are of especial value to them in aiding them to escape from this enemy . Sub-order I.—BASOMMATOPHORA . Pulmonata with an external shell . The head bears a single pair of contractile but not invaginable tentacles, at the base of which are the eyes . Penis at some distance from the female 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 lung . There is a veliger 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 spire, the internal partitions often absorbed and the aperture denticulated . Auricula .

Cassidula . Alexia .

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

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

foot short, operculated; marine . Amphibola . Fam . 4.—Siphonariidae . Visceral mass and shell conical; tentacles atrophied; head 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

columella; inferior pallial lobe thick; visceral commissure still 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,

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 . Fain. to . Ancylidae . Shell conical, not 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

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

Fam . 1.—Selenitidae . Radula with elongated and pointed

teeth, like those of the Agnatha; a jaw 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

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

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

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

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

borders of aperture thin and not reflected; radula with square teeth; genital ducts without accessory another, whilst two series of alternating rows cover the back and side . The body, otherwise circular in 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 muscular Mr oesophagus with a triradiate lumen, w4 which acts as a sucking pump and ....h,a ends in a funnel-valve projecting LT into the 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 tube on each side of the stomach; internally the tubes end in large flame-cells, and externally by small pores which 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 organs are the hairs and bristles and in some 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

fir Wissen- large, sometimes half the length of chalo zootogie, vol. x&x. p . 209, the mother; they are laid amongst by permission of Wilhelm Engel- maim. water weeds . The male reproductive Chaetonotus maximus, 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 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.

derma . Sch, Plates . (ii.) Fam . Chaetonotidae, with Se . Lateral bristles. bristles . Genera : Chaetonotus, Vb, Point of union of cili- Chaetura . ated 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

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