Although many of the species of Polygas tria are as large as the Rotifera, the structure of the latter is much more easily discernible, on account of the transparency of the lorica, or shield, in which they are enclosed, and the distinctness of their individual organs. The external covering, though always clear like crystal, has varying degrees of denSity, in some instances forrning a horny kind of case, insusceptible of movement, and, in others, a skin susceptible of transverse corrugations. Into this dense external membrane the animal is capable of drawing in its tail and rotatory organs; hence this class of animals has been called Systolides. In none of the species does there appear to be a deposit of earthy salts, either in the skin or other parts of the body.* This will account for the fdct, that few or none of the Rotifcra have been found in a fossilised state. Those forms alone of the Polygastria have been dis covered in the chalk and subsequent forma tions, which, in their living state, possess a si liceous or calcareous skeleton.
In the classification of the Rotifera we shall follow Ehrenberg, as no separate ar rangement of these creatures existed previous to his profound investigation of their struc ture ; and although other attempts have been made, since the appearance of his work, on the Infusoria, none of them seem better adapted for the purposes of further inquiry. At the same time we would, with the utmost diffi dence, express our doubts as to the correct ness of much of the terminology employed by Ehrenberg, implying, as it frequently does, views of the structure and functions of the parts of these animals which the facts them selves, so remarkably correctly observed, do not always seem to warrant. The following is a table of the eight families of Rotifera according to Ehrenberg :— It will at once be seen that this is an exceed ingly artificial arrangement ; for although the rotatory organs are the most striking external character of the Rotifera, the function they per form does not seem to be of that fundamental importance in the economy of the animal, so that a change in their form would be attended with corresponding changes in their general structure. In fact, in this arrangernent, forms are separated which are nearly related by the affinities of more important organs. In the next place, the families are arranged accord ing as they are naked (panzerlose), or loricated (gepanzerte). The condition of the integu ment here employed as a means of classifica tion, cannot be regarded as absolute ; and there are species which it would be difficult to refer to either group. Some of the species secrete around them an external tube, in which they dwell, as Stephanoceros (1g.
292.) and others, which is an entirely different thing from the hardened integument ,called by Ehrenberg the lorica, or shield, and yet these are classed as a loricated family. It is, however, but due to Ehrenberg to state that he is not unaware of the defects of this ar rangement, and that he has pointed out that both the structure of the alimentary canal, and even the teeth and jaws, would afford characters by which the species might be arranged. Dujardin, in a recent work on the Infusoria, proposes the four following fami lies : — I. Rotifers having the posterior part of their bodies ,fixed. Examples : Floscularia, Stephanoceros.
2. Rotifers having but one means of loco motion, that of the vibratile cilia, and which are consequently always swimmers. Exam ples : Ptygina, Lacumolaria, 3. Rotifers which have two modes of loco motion : one creeping like the leech, the other swimming as the last. This family includes the largest number of genera, as Brachionus, Dinoch&is, Pterodina, Salpina, Lepadella, Euchlanis, &c.
4. Rotifers without vibratile cilia, but which are supplied with nails, by means of which they walk. Examples ; Hydatina, 1Votom mata, Furcularia (3-c.
Dujardin, in h'is work, also objects to the characters on which Ehrenberg has consti tuted the various genera belonging to his eight families, these genera being principally determined by the presence or absence of little red spots, which Ehrenberg designates as eyes. (Fig. 292. a ; jig. 303. a ; fig. 296. a ; fig. 291. a ; fig. 298. a ; fig. 299. b.) The following is a description of the families adopted by Ehrenberg :— Fanzky 1. — ICT H YDIN A . Character.
Naked Rotifers, with a single continuous rotatory organ, not lobed at the mar,,ain.
In the genera Ptyg,ura arid Glenophora, the rotatory organ is circular, and serves as a means of locomotion. In Cluetonotus and Tally dium it is elongated, elliptical, band-like, and seated on the ventral surface. Chalonotus and /ethydhein possess a furcated foot, Pty gura and Glenophora a simple one. lethydium and Clicetonotus have a siinple conical intes tine, with a long thin cesophagus without teeth (?) Glenophora, a short cesophagus with two teeth; Plygura, a constricted stomach with three teeth (fig.288.). Pancreatic glands are only seen in Chotonotus and Ptygura. Cmeum, gall-ducts, and male sexual organs not observed. In two genera, the female sexual s:istem consists of an ovarium with a few large ova. The evidence of the existence of a nervous system is seen in the two large red frontal eyes of Glenophora. Chmtonotus has a hairy back.