—In the Infuso ria, whose minuteness places them at the lowest extremity of the animal kingdom, the organ of digestion has already attained such a development as to form the chief basis of their nomenclature.
One or two genera present us with a rare and exceptional condition :—viz. the absence of all traces of digestive cavity. Such are the parasitic Gregarina and °patina ; in whom, as in some of the Entozoa, digestion and absorption appear reduced to a simple phy sical process of endosmose, which carries the nutritious substances dissolved in the fluid medium they inhabit at_once into the mass of their corporeal juices.
The Polygastria possess a plurality of stomachs or internal sacs ; and the relations of these to the intestine, together with the con dition of the latter tube, subdivide this group into numerous families and genera. Thus rnany are named "anenterous," because they appear to be devoid of intestine. Of these the Monas termo--which has four or five globular stomachs, of rd--o-, -ath of an inch in diameter, appended immediately to its mouth—may be taken as the type. Others possess similar sacs in connection with a simple Intestine ; and are chiefly distinguished by the straight, curved, or wavy course of this canal,—or by the single or double character, and lateral or terminal position, of its apertures. Most of them devour a living prey of kindred Infuso ria ;—prehension being often visibly effected by cilia, the voluntary action of which carries a current of food into the mouth, or removes egesta by a simple reversal of the stream. And sotnetimes this act of ingestion ordinarily sufficient, are capable of being locally exhausted by the excessive demands of a particular class or species, aud renewed by au artificial supply.
is aided by a dental apparatus, in the shape of a hollow cylinder enclosing long teeth, — as in the genus Napula.
The Rotifera are so named from the cur rents produced by their prehensile cilia ; hich are here limited to groups surrounding the mouth of the animal.
Arany of them have an organ of mastica tion. This usually consists of three pieces : —each of the two facets of a kind of anvil being worked upon by the rough or toothed terminal surface of a recurved jaw, the longer limb of which receives a muscle at its extremity,.
The intestinal canal generally exhibits a pharyngeal enlargement, which is followed by a narrow "cesophagus," of varying length, ending in a wider " intestine.'" In the Gastcrodela a dilatation, called a stomach, pre cedes the intestine. In the Rotifer vulgaris and others, an almost globular enlargement of the narrow canal is so immediately followeil by the constricted cloaca, as to have been com pared to a large intestine. The organ of digestion is also often complicated by the presence of blind tubes ; which vary, not only in number and size, but also in posi tion, and possibly in import. Thus they may open, either into an uniform and narrow canal, or into the commencement of the intestine, or into the presumed gastric dilatation ;—or, finally, as in the Diglerna lacustris, a set of' such tubes may occupy both of these latter situa tions.
The various members of the order Entozoa are grouped together in obedience to a classi fication which is here and there arbitrary and anomalous, but in the main both natural and useful. It offers three chief varieties of the digestive organ, all of which are very inte resting.
a. In many — as in the Echinococci and their congeners--no trace of a special digestive cavity is present. Without mouth, stornach, or intestine, the creature floats free in the cavity of its enclosing cyst, or buries its barbed head in the tissues of a living habitation;—whose juices, thus brought into relation with its exterior, are applied to its nourishment by what s.eems to be rather a process or endostnose than of digestion pro perly so called.
0. In other genera belonging to the Cestoid and Trernatoid divisions there is, how ever, a canal, which is apparently related to digestion, and the main features of which—repetition and ramification—may be represented by. the Teenia and Distoma respectively.