In many of the Rotifers, more especially the Philodinma, the tail is employed for the purposes of progression. In order to effect this, it is supplied with distinct muscular bands (fig. 293. e). Muscular fibres have also been described by Ehrenberg surrounding the oesophagus, and apparently assisting the jaws in their movements, in Conochllus (fig. 289.), Pleurotrocha, and other species.
Digestive systenz.— This apparatus is per haps more highly developed than any other part of the Rotifera. An oral and an anal orifice indicate the commencement and end of this system. It is furnished with jaws and teeth, an oesophagus, sometimes a distinct sto mach, and various forms of intestinal glands. At the commencement of this system, we find a distinct masticatory apparatus, which consists generally of two semicircular pieces, to each of which is attached one or more teeth, which act upon a central plate Cifg. 300. ; fig. 299. e;.fig. 289. a, aLfig.296. b ;fig. 292. d ; fig. 291. b). To the semicircular pieces are attached some of the longitudinal muscles, which, by their action, cause the teeth to work upon the central plate. The general structure of the jaws is, in all instances, the same, but the number of processes, called teeth, varies considerably. Sometimes no such processes are discoverable, as in Cyphonautes; in others, there is but one tooth on each side, as in spe cies of Synchteta, Diglena, Lepadella, and Mo nostyla. Some have two on each side, as Mo
nocerca, Mastigocerca, and Rotifer ( fig. 301.). Others, again, present three, four, or several teeth on each side, as in Brachionus brevissi nzus (fzg.300.), Triarthra longiseta (fig. 297.), Brachionus pala (fig. 296.). Such are the cha racters afforded by the teeth in this family, that Ehrenberg gives an arrangement of the earlier observers for the action of a heart. Although, in most instances, the food is brought to the jaws of the animal by the rotatory apparatus, yet we have often ob served, in species of Brachionus, that they have the power of projecting their jaws be yond the margin of the teguinentary mem brane, and bringing them immediately in con tact with the substances on which they are feeding.
The form of the alimentary canal varies : it is sometimes a simple tube, as in Dinocharis paupera (fig. 302. b). In many species, an enlargement of the middle portion takes place from constriction of the canal, above and below, forming a kind of stomach, as in No tommata myrmeleo (fig. 303. c), Brachionus pala (fig. 296.). Sometimes there are two en genera into orders, according to the absence or presence and number of the teeth, as follows :—