COMPARATIVE ANATOMY. 
frog, although it is nine times as long as the 
space between the mouth and the anus in 
the tadpole. 
The alimentary canal of some fishes is 
continued straight from the mouth to the 
aiius, and does not, therefore, equal the 
length of the body. The lamprey, skate, 
and shark are thus circumstanced. 
Most birds have two caeca, which are 
longer in the gallin® than in the carnivo- 
rous tribes. Tlie rectum ends in a part called 
the cloaca, which is a large membranous 
bag, containing also the termination of the 
ureters, that of the oviduct, the Vasa defe- 
rentia, and of a membranous bag of un- 
known use, called bursa fabricii. This also 
holds the penis where there is one. 
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 sup- 
posed by some to be an animal consisting 
eniirely of a stomach. Doubts, however, 
have lately been raised whether or no this 
be really an animal. Even if it were 
allowed that these bags are animals, 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 generation. 
The hydatid, which is more frequently 
found in animals, which possesses a head 
and mouth like the taenia, enabling it to at- 
tach itself to parts, and whieh can be seen 
to move when placed in warm water, is 
generally allowed to possess an independent 
vitality. But whether the bag of water, 
which forms its body, be a stomach, is cer- 
tainly doubtful. 
The most simple form of an alimentary 
cavity exists in the common fresh water 
polype (hydra). It appears to be excavated 
in the substance of the body, and has a sin- 
gle opening situated in the centre of the 
space surrounded by the tentacula. The 
nutritive matter soaks immediately into 
the body, and imparts its colour to the 
animal. 
The large masses of gelatine, called me- 
dusae, which resemble in form mushrooms, 
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 fluid over the body. Sometimes 
the stomach has a simple opening; 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 con- 
stitutes a remarkable analogy to the roots 
of trees; and Cuvier has formed a new 
genus under an appellation derived from 
this comparison, viz. the rhizostoma, from 
a root, and roy-a a mouth. 
The star-fish (asterias) has a membranous 
cavity in the centre of its body, communi- 
cating externally by a single opening ; tvto 
canals extend from tins into each of the 
branches, or as they are sometimes called 
the fingers of the animal, where they sub- 
divide and form numerous blind processes. 
The tape-worm (taenia) has a small canal 
running on each side of its body ; the two 
tubes are joined together by transverse pro- 
ductions at each joint. 
The ascaris lumbricoides (round-worm) 
has a simple canal running from one extre- 
mity of the body to the other. 
The leech (hirudo sanguisuga, or medici- 
nalis) has a short oesophagus and a very 
large stomach, divided by numerous mem- 
branous septa, which are perforated in the 
centre. It has been generally supposed 
that this animal has no anus; but Cuvier 
says that it possesses a very small one. 
(L^Qons d’Anat. Comp. tom. 4. p. 141) 
Dumeril, on the contrary, denies its exis- 
tence. (Zoologie Analytique, p. 298.) 
The common earth-worm (lumbricus ter- 
restris) has a long canal, divided by several 
partitions. 
The aphrodite aculeata has an intestine 
running according to the length of the body, 
and sending off on each side several blind 
processes, which enlarge at their termina- 
tion. 
In the proper mollusca, besides the sto- 
mach, which has been already noticed, there 
is an intestine, seldom of considerable 
length, making some turns in its course ; it 
passes in all the acephalous mollusca 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 vegetables ; while 
the contrary characters indicate animal 
food. 
Great difference is found, in some in- 
stances, between the larva and the perfect 
insect. The voracious larvae of beetles 
