PISCES. 
of the skeleton, and a series of deep alveoli 
is excavated in each of its lateral margins (fig. 
518). 
The teeth which are lodged in these sockets 
are elongated, compressed in the same plane 
as that of the body of the saw, and their edges 
converge to a sharp point, which is situated a 
little behind the axis.of the tooth. Each ros- 
tral tooth is solid, its base being slightly con- 
cave and porous like the section of a cane, but 
the pores are finer and more numerous. The 
walls of the socket are formed by ossification 
of the rostral cartilages to an adequate extent ; 
but as unnecessary weight under any circum- 
stances, but especially at the fore-end of the 
fish, would be’a cumbrous impediment to its 
motions, the spaces intervening between the 
sockets are hollow and filled with a gelatinous 
medulla. A large vascular canal traversed by 
branches of the facial artery and of the second 
division of the fifth pair of nerves enclosed 
in a cellular gelatinous tissue, runs parallel 
with the axis of the saw along the back part of 
the alveoli, and supplies the materials for the 
increase of the teeth, which are not shed and’ 
renewed like the maxillary teeth, but grow 
with the growth of the body by constant 
addition of fresh pulp-material progressively 
ossified at their base. 
(Esophagus.—Owing to the extreme short- 
ness of the passage between the cavity of the 
mouth and the stomach, the length of this tube 
is extremely limited in the whole race of Fishes; 
nevertheless, occasionally its boundaries are 
well marked, and its structure sufficiently dis- 
tinct from that of the alimentary canal to entitle 
this part of the digestive apparatus to a brief 
notice. Its walls are generally strong and 
muscular for the purpose of passing the food 
into the stomachal cavity, but more especially 
so in those ‘voracious Fishes which swallow 
dense and indigestible shells of various kinds, 
or that are subject to have their stomachs 
loaded with the hard bones of digested fishes, 
all which materials are in such instances regur- 
gitated and thrown out of the mouth much after 
the manner of the “castings” of the Hawk or 
Owl among birds of prey. In a few races, how- 
ever, the esophagus exhibits peculiarities of 
structure that are remarkable. Thus in the 
Torpedo and others of the Ray genus, there is 
a very thick layer of a soft and semi-gelatinous 
substance interposed between the lining mem- 
brane and the muscular coat, the use of which 
it is by no means easy to conjecture. In the 
Sturgeon the mucous membrane of the gullet 
‘is prolonged into transverse valvular folds, 
_analogous in their nature to the conical pro- 
cesses found in that of the Turtle. But the 
most striking example of a valvular apparatus 
‘Situated in this part of the digestive tube is met 
with in the Sharks (fig. 519), in which race 
of Fishes the termination of the esophagus is 
indicated. by a great number of long fleshy 
‘stems, which divide and subdivide into very 
numerous branches, and thus form a dense and 
prominent fringe hanging loosely downwards 
towards the stomach, in such a way as to allow 
anything swallowed to pass freely im that direc- 
981 
tion, but by the interlacement of the fringes 
effectually preventing anything from returning 
towards the mouth. This structure is most 
conspicuously seen in the great Shark ( Squalus 
maximus ),in which species, owing to the com- 
parative smallness of the teeth which arm the 
Jaws, it is extremely probable that many Fishes 
are swallowed alive and might retain their vita- 
lity sufficiently long to struggle back again out 
of the stomach of their devourer, did not this 
strange gate effectually bar their progress. 
Stomach.—This viscus is, in the generality of 
species, a musculo-membranous bag of very 
simple structure and of variable shape in diffe- 
rent genera. Sometimes, however, its muscular 
walls are sufficiently strong to perform to a certain 
extent the functions of a gizzard: this seems to 
be the case, for example, in the Gillaroo Trout 
(Salmo fario), and to a far more remarkable 
extent in the Mullets (Mugil), in which the 
esophagus terminates inferiorly in a deep cul- 
de-sac that serves the purpose of a crop. The 
muscular stomach or gizzard opens from one 
side of this cesophageal bag, to which it is 
attached at right angles. Its shape is pyriform, 
the narrow end being directed towards the in- 
testine, and its muscular walls nearly half an 
inch thick. Internally it presents numerous 
longitudinal folds, all covered with a thick cu- 
ticular lining, and evidently the whole appara- 
tus is very analogous to the true gizzard of 
a granivorous bird. It was John Hunter’s 
opinion that neither of these can be justly 
regarded as gizzards, since they want the most 
essential characters, namely, a power and mo- 
tion fit for grinding, and a horny cuticular 
lining. According to Professor Owen, how- 
ever, the latter structure exists in the gizzard of 
the Mullet as a distinct layer of rough and 
easily separable cuticle. The stomach of the 
Gillaroo or Gizzard-trout is certainly, as de- 
scribed by Hunter, more globular in its shape 
than that of most Fisl, and endued with suffi- 
cient strength to break the shells of small shell- 
fish, which is probably accomplished by having 
more than one in the stomach at a time, and 
also by taking pretty Jarge and smooth stones 
into the stomach, which will answer the pur- 
pose of breaking, but not so well that of grind- 
ing; nor will they hurt the stomach, as they are 
smooth when swallowed ; but this stomach can 
scarcely possess any power of grinding, as the 
whole cavity is lined with a fine villous coat, 
the internal surface of which appears every- 
where to be digestive. The common stream- 
trout, which only differs from the “ Gillaroo”’ 
in having the walls of its stomach not so thick 
by two-thirds as the other variety, likewise 
swallows smooth round stones wherewith to 
crush the shell-fish which it occasionally de- 
vours. 
Intestinal canal.—The intestines of Fishes 
present as many variations as regards their rela- 
tive length when compared with that of the 
body, as do those of any ’of the higher Verte- 
brata, but their general arrangement’is of a 
much more simple character. In the Flying- 
fish ( Exocetus ), indeed, it presents the sim- 
plest possible form, being merely a straight 
