ZOOLOGY. 849 
tion, that, as the skeleton constitutes the external covering of 
the body, so they are situated within the bony cylinders of 
the body and limbs, to the motions of which they contribute. 
Thus in crabs, each of the five legs on each side consists of 
five joints, to which, in the first pair, a sixth is superadded, 
being connected with the fifth articulation, and assisting to 
form the claws or nippers. Internally, each leg is prolonged 
under the dorsal plate in the form of a horny expansion, 
supporting the gills, and coinciding both in form and func¬ 
tion with a scapula. Each joint of the leg contains an ex¬ 
tensor and flexor muscle for the joint beyond it, the muscles 
of the first joint being attached to the horny plate within 
the thorax, in the same manner as the muscles of the upper 
arm in other animals are attached to the scapula. The mus¬ 
cles of the first pair of legs are particularly strong, as these 
are larger than the others, and constitute the nippers; the 
thumb of which, if we may call it so, is moved with greater 
force by strong muscles filling the spacious cavity of the 
fifth phalanx. Besides the muscles of the limbs, those of 
the tail also, are very considerable, forming a superior and 
inferior layer of complicated bundles of fibres, which are 
separated by the interposition of the rectum in its passage 
through the tail. 
A great diversity exists in the class insecta. In the centi¬ 
pedes, for instance, we still find the perfect shape of the 
vermes; and even their legs are but little more than the move- 
able bristles of those animals. Others, again, as the scor¬ 
pions and spiders, rather resemble crabs ; their organs of lo¬ 
comotion, in point of organization, coinciding remarkably 
with the Crustacea. But, neither in these nor in other insects, 
do we find a calcareous shell, the external moveable ske¬ 
leton being uniformly composed of horny plates. As to the 
external organs of motion, the structure of the legs, the num¬ 
ber and position of which are liable to many varieties, coin¬ 
cides pretty closely with crabs. The wings of insects are 
attached to the pectus, and moved by muscles placed within 
it. 
Before a single step can be taken with regard to the lo¬ 
comotive system in the remaining classes, it will be necessary 
to give an account of their skeletons. 
The skeleton in fishes is extremely various. They agree 
together, however, on the whole, in having a spine, which 
extends from the cranium to the tail-fin ; and in having the 
other fins, particularly those of the thorax and abdomen, ar¬ 
ticulated with peculiar bones destined to that purpose. They 
have in general many bones unconnected with the rest of the 
skeleton. 
The cranium in several cartilaginous fishes (in the skate for 
instance) has a very simple structure, consisting chiefly of 
one large piece. In the bony fishes, on the contrary, its 
component parts are very numerous; amounting to eighty in 
the head of the perch. Most of the latter have a more or 
less moveable under-jaw. 
Great variety in the structure of the teeth is observed in 
this class. Some genera, as the sturgeon, are toothless. 
Their jaws, which are distinct from the cranium, form a 
moveable part, capable of being thrust forwards from the 
mouth, and again retracted. 
Those fishes which possess teeth, differ very much in the 
form, number, and position of these organs. Some species 
of bream (as the sparus probato-cephalus) have front teeth 
almost like those of man; they are provided with fangs, 
which are contained in alveoli. In many genera of fishes 
the teeth are formed by processes of the jaw-bones covered 
with a crust of enamel. In most of the sharks, the mouth 
is furnished with very numerous teeth for the supply of such 
as may be lost. The white shark has more than two hundred, 
lying on each other in rows, almost like the leaves of an 
artichoke. Those only which form the front row have a 
perpendicular direction, and are completely uncovered. 
Those of the subsequent rows are, on the contrary, smaller, 
have their points turned backwards, and are covered with a 
kind of gum. These come through the covering substance, 
and pass forward when any teeth of the front row are lost. 
Vol. XXIV. No. 1677. 
It will be understood from this description that the teeth in 
question cannot have any fangs. 
The saw-fish only (squalus pristis) has teeth implanted in 
the bone on both sides of the sword-shaped organ with which 
its head is armed. 
In some fishes the palate, in others the bone of the tongue 
(as in the frog-fish), in others (as in several of the ray-kind) 
the aperture of the month forms a continuous surface of tooth. 
In the long-shaped fishes with short fins, the spine con¬ 
sists of a proportionally greater number of vertebrae; of 
which the eel, for instance, has more than one hundred, and 
some sharks even more than two hundred. The main 
piece, or body, as it is called, of these vertebrae, is of a cy¬ 
lindrical figure, with a funnel-shaped depression on both 
surfaces, and concentric rings, which are said to vary in 
number according to the age of the animal. The spinal 
marrow passes above these, in a canal formed at the roots of 
the spinous processes. 
The ribs are articulated with what are called the dorsal 
vertebrae in most of the spinous fishes; but in some they are 
without this connexion, and in the cartilaginous fishes proper 
ribs cannot be said to exist. 
Of the peculiar bones which serve as a basis for the fins, 
that of the pectoral fin may be compared to the scapula, 
and that of the abdominal in some measure to the os inno- 
minatum. 
Fig. 30 is an example of the skeleton in fishes. It is the 
cyprinus alhurnus : a, the occipilal bone; b, the parietal 
bone; c, the frontal bone; d, the nasal bone; e, the supe¬ 
rior maxillary bone, moveable and arched like the inferior 
maxilla ; f the intermaxillary bone, likewise moveable and 
arched; g, the inferior maxilla; h, fossa for the olfactory 
organ ; i, arch of the zygoma; i*, the orbit; k, the os qua- 
dratum; /, the’operculum ; m, the lingual bone; n, the 
lingual cartilage; o, the radii of the branchial membrane; 
р, bones of the shoulder; q, scapular appendage, by means 
of which the osseous belt of the shoulder is attached to the 
cranium ; r, a rudiment of bones of the arm, supporting the 
radii of the thoracic fins; s, furcular bone ; t, thoracic fin ; 
u, abdominal fin ; u*, a rudiment of bones of the pelvis, to 
which the abdominal fin is attached ; v, dorsal fin; w, anal 
fin; x, caudal fin, supported by, y, the radiating laminae of 
the last caudal vertebrae. 
Fig. 31. Head and thorax of the long-nosed shark:— a, 
the superior maxillary bone; g, the inferior maxilla ; n, the 
lingual cartilage; p, bones of the shoulder; t, thoracic fin; 
с, the sternal cartilage; d, the branchial arches, or thoracic 
ribs; e, the external cartilaginous laminae and arches, ap¬ 
pended to the proper branchial arches. 
The muscles of this class are distinguished from those of 
animals which breathe by means of lungs, not only by re¬ 
ceiving a smaller supply of blood, and consequently being 
of a paler colour, but also by their disposition in layers, and 
by the uniformity of their substance, which in general is de¬ 
stitute of tendinous fibres. This structure, together with the 
number and bulk of their muscles, is well calculated to sup¬ 
port that great expenditure of strength and exertion, which 
is a necessary consequence of the peculiar abode, and whole 
economy of these animals. 
Fig. 30*. The muscles of the cyprinus alburnus, as they 
appear after the common integuments are removed : k, oper¬ 
culum ; o, an elastic ligament for raising the eye ; r, a mus¬ 
cle which, with the following one, depresses the superior 
maxilla; s, a retractor of the angle of the mouth ; t, ele¬ 
vator of the inferior maxilla; u, depressor of the same part; v, 
abductor of the pectoral fin, (h); x, the great lateral muscle, 
composed of numerous bundles of fibres; y, muscular fasci¬ 
culus of the caudal fin, (c); d, elevator of the dorsal fin, (e) ; 
muscular fasciculus of the abdominal fin, (g). 
On the skeleton of amphibia .—The testudines, (turtles 
and tortoises) whose whole skeleton, and indeed whose whole 
body has a very peculiar structure, are entirely toothless; 
they have, however, a kind of os intermaxillare in the upper 
jaw. The horny covering of the jaws, particularly the upper 
9 E one. 
