GASTROTRICHA, a small group of fairly uniform animals which live among Rotifers and Protozoa at the bottom of ponds and marshes, biding amongst the recesses of the algae and sphagnum and other fresh-water plants and eating organic débris 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 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 everted. This leads to a muscular oesophagus with a triradiate lumen, which acts as a sucking pump and ends in a funnel-valve projecting 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.
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| From Zeitschrift für Wissenschaft. Zoologie, vol. xlix. p. 209, by permission of Wilhelm Engelmann. | |
| Chaetonotus maximus, Ehrb., ventral side. (After Zelinka.) | |
| Bo, | Bristles surrounding the mouth. |
| ds, | Dorsal bristles. |
| hCi, | Posterior lateral cilia. |
| Ke, | Cuticular dome. |
| Mr, | Oral cavity. |
| lT, | Lateral sensory hairs. |
| Pl, | Cuticular plates. |
| Sa, | Dorsal bristle of the basal part. |
| Sch, | Plates. |
| Se, | Lateral bristles. |
| Vb, | Point of union of ciliated tract. |
| vCi, | Anterior group of cilia. |
| vS, | Ventral bristles of the basal part. |
The two ovaries lie at the level of the juncture of the stomach and rectum. The eggs become very large, sometimes half the length of the mother; they are laid amongst water weeds. The male reproductive system is but little known, a small gland lying between the ovaries has been thought to be a testis, and if it be, the Gastrotricha are hermaphrodite.
Zelinka classifies the group as follows:—
Sub-order 1.—Euichthydina with a forked tail.
(i.) Fam. Ichthydidae, without bristles. Genera: Ichthydium, Lepidoderma.
(ii.) Fam. Chaetonotidae, with bristles. Genera: Chaetonotus, Chaetura.
Sub-order 2.—Apodina, tail not forked. Genera: Dasydytes, Gossea, Stylochaeta.
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.
Literature.—A. C. Stokes, The Microscope (Detroit, 1887–1888); C. Zelinka, Zeitschr. wiss. Zool. xlix., 1890, p. 209; M. Voigt, Forschber. Plön. Th. ix., 1904, p. 1; A. Giard, C. R. Soc. Biol. lvi. pp. 1061 and 1063; E. Daday, Termes. Fuzetek. xxiv. p. 1; F. Zschokke, Denk. Schweiz. Ges. xxxvii. p. 109; S. Hlava, Zool. Anz. xxviii., 1905, p. 331. (A. E. S.)