ln the Erhinvs, the anus generally opens on the upper or opposite surface of the body. Many of this genus have a complex masti catory organ, which is acted upon by powerful muscles. The first part of the canal opens into an intestine of much larger diameter, opposite to a blind dilatation very like the human czecum. The intestine is coiled twice around the inner surface of the shell ; the second coil reversing the direction taken by the first, and both exhibiting a sinuous course. Its width tapers away to the anus. Its struc ture is delicate and transparent; it possesses a mucous membrane, and longitudinal and trans verse fibres ; and it exhibits an intestinal vein, which is especially rnarked towards the termi nation of the canal.
In the vermiform Holothurku the canal forms a kind of Z in the abdominal cavity;— passing first backwards, then forwards, and again backwards to its posterior extremity. The first part is wider and stronger than the rest, and its more glandular mucous tnembrane presents longitudinal folds which terminate in a slight circular one. Such a structure causes this dilatation to be regarded as a stomach. The narrowing intestine often ter minates in a large oval cloaca, into which open two branching cca.# The Annelida form a class of animals so diverse in nature and structure, that it is diffi cult to include all the varieties of their diges tive apparatus within a mere brief sketch. The canal always possesses a distinct mouth and anus, which occupy the opposite ends of the more or less elongated and cylindrical body. Prehension is generally aided by teeth, which, as in the Leech, perforate the skin of their prey ; while in others—as in sorne of the Errantes—it is effected solely by -a proboscis. In many of the marine Errantes the intes tinal canal is simple. In the Lumbrici the canal soon dilates into a membranous pouch, which is followed by a thicker and more muscular portion, supposed to be a gizzard. In some genera, this part of the tube is com plicated by being produced into pouches. These are either numerous, as in the Leech; or few, as in some kindred genera. Fi nally, in the Earth-worm, they are reduced to mere constrictions of the canal ; while in the Aphrodita, they are developed into tubes, which expand, divide, and terminate as al most globular pouches. Clusters of glandular follicles, which are supposed to be biliary, open into the posterior half of the complicated canal of the leech just alluded to : and analo gous structures are found in other gencra. In the Earth-worm, there is a singular ap paratus, the typhlosole. This is a blind tube, which occupies almost the whole length of the canal, being attached to its dorsal aspect, and projecting into its cavity. Its interior surface is folded and villous. The whole structure appears to be connected with a kind of chy lous absorption.
The alitnentary canal of the Epizoa differs from that of the cavitary Entozoa, in being generally surrounded by a glandular mass, the function of which is probably hepatic. The
Cirripeda have prehensile jaws, and a ter minal mouth and anus. In some, the canal has a gastric dilatation. IIepatic follicles, similar to those already described, occur here also. And St. Ange and Serres have fOund a tube analogous to the typhlosole.
The digestive tube of the Crustacea may be reduced to two chief forms, which corre spond with other differences in the nature and structure of their possessors. Thus in those lower Crustaceans which are suctorial and para sitic, the canal is a very simple one. A proboscis conceals a pair of lancet teeth, and is followed by a straight intestine, around which are clustered a dense mass of follicles, supposed to constitute a liver. The higher Crustaceans possess a complicated apparatus of forceps and jaws. A short cesophagus leads to a large spherical cavity, which occupies the head of the animal, and which, although sometimes called a stomach, contains hard structures tbat render it analogous to a gizzard. A well marked constriction separates this organ from the intestine, which is sometimes simple and nearly straight, sometimes divided into two portions distinguished by a projecting valve. The liver is conglomerate, and divided into lobules. Rarely, one or two cmcal tubes are also present.
The alimentary canal of the Insect offers what are rather varieties of development than any regular transition, such as we have remarked in some of the preceding orders :— varieties which the metamorphosis of the larva at present seems to conaplicate instead of ex plain.
In the larva, the canal is comparatively simple, and somewhat approaches the condi tion seen in the lowerAnnelidet; being a straight tube, with a mouth and anus at opposite ends of the body. In many Hymenopterous larvm, the latter aperture is absent, In others it is only developed towards the end of this stage of life, when an excrement—or meconiunt as we may perhaps call it—is for the first time expelled. But though such an intestine might seem to resemble that of the anen terous Infusory, or the hydriform Polyp, we must recollect that it differs from these in the important fact of its not being used for the double purpose of ingestion and egestion. The complications of the above simple canal relate chiefly to its subdivision,and to the addi tion of blind tubes. A gastric dilatation is the first to appear ; its extremities then become constricted, and its calibre enlarged. An ceso phagus, a crop or ingluvies, a small and a large intestine, may also be added. Sometimes the supposed stomach is transversely divided into two cavities, and complicated by short cca. In other instances, longer tubes open into the same part of the canal. While in others, they open into the intestine below this point; and are hence presumed to be biliary.