For example, in the Tape-worm, a minute mouth opens into a slender tube, the bifurca tions of which reach the margins of the body where this begins to assume its regular jointed form. From hence onwards the canal rnight be compared to a ladder, with rungs at the fore and aft extremity of each joint, at the right angles of which its longitudinal and transverse branches unite. It is probable that these canals possess valves. But whether they- have any constant or valid terminal ori fices seems doubtful.
Many species of Distoma or Fluke may be recrarded as types of an arborescent or rami fie% digestive tube. From a mouth vvhich is suctorial—and sometimes visibly muscular —a canal passes backwards, to divide into two large branches. These run along the margins of the oval and flattened animal, giving off other branches ; from which proceed a final series of anastomosing twigs.
y. In many creatures closely allied to the preceding by conformation and habits, this ramified canal is reduced to its primary bi furcations, the ends of which are sometimes slightly dilated. Occasionally there is an enlargement, which has the situation of a pha rynx ; and which, in a few instances, encloses an apparatus perhaps masticatory. In the genus Diplostomum and others, a distinct set of vessels, which occupies the itnmediate neigh bourhood of the intestine, has been supposed to represent a chyliferous or vascular system.
As regards these latter forms of digestive apparatus, it may be conjectured, that the ramikation witnessed in the Tcenia is re ferrible, not so much to that mere vege tative repetition of' similar structures which affects the whole animal, as to a merging of the digestive in the circulatory function. In any case, the more simple form of tube iast mentioned appears rather akin to an advance, than to a retreat, of development; while it sometimes visibly coincides with the appearance of a new system of canals, con nected with the circulation of a proper nu trient fluid.
In the Nematoid Entozoa, the alimentary canal is generally a straight tube, which oc cupies the axis of the vermiform animal, and opens at its extremities. In most genera — as in the Trichina, Tricocephalus, Ascaris, Strongylus, and others — it widens posteri orly ; where it often experiences a further dilatation, which only ceases near the anus. Rarely, other indications of separation are added : — an cesophageal dilatation, as in the Ascaris lzimbricoidcs; or an enlargement cor responding in position to a stomach, as in the Linguaiula and Fdaria.
Rudiments of the organs accessory to di gestion have also been detected. Blind tubes opening into the canal near its mouth are found in several genera : and the position of these has sometimes led to their being re garded as salivary. While rarely there is a tube which opens into the intestine in the situation of a biliary organ.
In the mode of attachment of their diges tive canal, this division of the Entozoa offers a marked contrast with the preceding. In the Sterelmintha (or solid worms) the tube is scarcely distinguishable from the mass of the body. While in these Ccelelmintha (or hollow Nematoid worms), it is suspended from the wall of the belly by filamentous processes. And though such an acquisition of an abdotninal cavity is no doubt partly referrible to the isola tion demanded by the organs of locomotion, yet not only does this itself imply a general advance of development, but it is actually accompanied by a curious structure, which is apparently connected with nutrition, and pos sibly renders the cavity of the abdomen the receptacle of a kind of chyle. Its more perfe2t form in the Ascaris lunzbricoides may be briefly described as consisting of a series of pyriform processes, the peduncles of which are seated unmediately upon the mesenteric filaments previously alluded to, and which project freely into the abdominal cavity, so as to be sur rounded by the serum and grey transparent substance that fills this space. Their shape resembles that of the villi of higher animals; and their size increases towards the median line of the body.
The alimentary canal of the Polyp exhibits so wide a range of development. that while by one extreme it approaches that of the sim plest anenterous Infiisoria, by the other it attains a complexity akin to that of the highest In ver tebrata.
The Hydra is little more than a stomach or sac, fixed by a sucker at its closed extre mity, and having at its other end a mouth gur rounded by prehensile tentacles. Digestion is, however, energetic. The living prey, which is paralysed by the deadly grasp of the ten tacles, undergoes a rapid solution in this sto mach, while its colours often visibly mix with those of the parietes common to the organ and the animal; and finally, its excrementitious residue is speedily rejected by the same orifice through which it previously entered.