ALIMENTARY CANAL OF THE LOWER, ORDERS.
The simple globular hydatid, which is frequently found in the different viscera both of man and quadrupeds, has been supposed by some to be an animal con sisting entirely of a stomach. Doubts, however, have been lately raised, whether or no this be really an animal. Even if it were allowed that these bags are ani mals, it does not follow that their cavity is a stomach ; and the attachment of the young to the sides would rather justify us in considering it as the organ of gene ration.
The hydatid, which is more frequently found in animals which possess a head and mouth like the twnia, enabling it to attach itself to parts, and which can be seen to move when placed in warm water, is generallyallowed to possess an indepen dent vitality. But whether the bag of water, which forms its body, be a sto mach, is certainly doubtful.
The most simple form of an alimentary cavity exists in the common fresh water polype (hydra). It appears to be exca vated in the substance of the body, and has a single opening situated in the cen tre of the space surrounded by the ten tacula. The nutritive matter soaks im mediately into the body, and imparts its colour to the animal.
The large masses of gelatine, called medusx, which resemble in form mush rooms, and are found floating in the sea, have a somewhat similar structure. A stomach is hollowed out in the pedicle ; and vessels, commencing from its cavity, convey the nutritious food over the body. Sometimes the stomach has a simple open ing; in other cases there are branching tentacula, on which canals commence by open orifices ; these unite together to form larger tubes, and the successive union of these vessels forms at last four trunks, which open into the stomach, and convey the food into that cavity. This very singular structure constitutes a re markable analogy to the roots of trees; and Cuvier has formed a new germs, under an appellation derived from this comparison, viz. the rhizostoma, from 'etC12, a root, and soma, a mouth.
The star-fish (asterias) has a membra nous cavity in the centre of its body, communicating externally by a single opening ; two canals extend from this into each of the branches, or, as they are some times called, the fingers of the animal, where they subdivide and form numerous blind processes.
The tape-worm (tmnia) has a small ea nal running on each side of its body ; the two tubes are joined together 'by trans verse productions at each joint.
The ascaris lumbricoides (round-worm) has a simple canal running from one ex tremity of the body to the other.
The leech (Iiirudo sanguisuga, or me dicinalis) has a short (Esophagus and a very large stomach, divided by nume rous membranous septa, which are per forated in the centre. It has been ge nerally supposed that this animal has no anus ; but Cuvier says that it possesses a very small one. Lecons d'Anat. Comp. torn. iv.p. 141.) Dumeril, on the contrary, denies its existence. (Zoologie Analyti que, p. 298.) The common earth-worm (lumbricus terrestris) has a long canal, divided by se veral partitiks.
The aphrodite sculeata has an intestine running according to the length of the body, and sending off on each side seve ral blind processes, which enlarge at their termination.
In the proper mollusca, besides the sto mach, which has been already noticed, there is an intestine, seldom of considera ble length, making some turns in its course ; it passes in all the acephalous molluscs through the heart.
The intestinal canal of insects varies very much in the different genera and species. It may be stated, on the whole, that a long and complicated intestinal tube denotes that the insect feeds on vegeta bles ; while the contrary characters indi cate animal food.
Great difference is found, in some in stances, between the larva and the per fect insect. The voracious larvx of bee tles (scarabmi) and butterflies, have intes tines ten times as large as the winged in sects which are produced from them.
In the dragon-fly (libellula,) which is very carnivorous, the intestine is not long er than the body. There is a small but muscular stomach.
The orthoptem (which class contains the locusts, &c. well known for their de structive powers) have a long and com plicated alimentary apparatus. They have first a membranous stomach. This is suc ceeded by another cavity covered inter nally with scales or teeth, and possessing a very thick muscular coat ; in short, a true gizzard. Round the end of this the cmcal processes are attached. There is, lastly, an intestinal canal, differing in length and diameter.
The. alimentary canal runs straight along the body in the trustacea, and is uniform in its dimensions, excepting the stomach.