Gastrotricha - Encyclopedia




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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 onesixtieth 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, ~kCi LT Chaetonotus maximus, Ehrb., ventral side. (After Zelinka.) Bo, Bristles surrounding the mouth.

1T, Lateral sensory hairs.

ds, Dorsal bristles.

hCi, Posterior lateral cilia.

Ke, Cuticular dome.

PI, Cuticular plates.

Sa, Dorsal bristle of the Sch, Plates.

Mr, Oral cavity.

Se, Lateral bristles.

Vb, Point of union of cili basal part.

ated tract. phrodite.

lows

forked tail.

bristles. Genera: Ichthydium, Lepidoderma. bristles. Genera: Chaetonotus, Chaetura. (ii.) Fam. Chaetonotidae, with Zelinka classifies the group as fol Sub-order I. - Euichthydina with a Sub-order 2. - Apodina, tail not (i.) Fam. Ichthydidae, without forked. Genera: Dasydytes, Gossea, vCi, Anterior group of cilia. Stylochaeta. vS, Ventral bristles of the The genus Aspidiophorus recently basal part. 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. Plon. Th. ix., 1904, p. 1; A. Giard, C. R. Soc. Biol. lvi. pp. 106r and 1063; E. Daday, Termes. Fuzetek. xxiv. p. I; F. Zschokke, Denk. Schweiz. Ges. xxxvii. p. 109; S. Hlava, Zool. Anz. xxviii., 1905, p. 33 1. (A. E. S.)

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