PISCES. 
straight, parallel, and fleshy, passing from the 
superior margin of the upper jaw to the infe- 
rior margin of the lower. 
The third presents a very singular arrange- 
ment, having its fibres interlaced in a very re- 
markable manner. These, however, may be 
divided into three principal masses, two of 
which are anterior aiid one posterior.* 
One of these masses is situated in front of 
and above the upper jaw near the commissure, 
It is attached to its superior margin, and runs 
obliquely to join the external edge of the second 
mass. This latter occupies nearly the same 
ition relative to the lower jaw; it passes be- 
ind the other and is conjoined with it exter- 
nally. The third or posterior mass is derived 
from the end of the upper jaw, and joins the 
hinder margin of the second. All these fibres 
so singularly interlaced co-operate in holding 
the mouth closely shut when the Skate has 
seized its prey. 
_ Lastly, there are two very long muscles de- 
rived from the spine, which pass between the 
palate and the cranium to be inserted into the 
upper jaw. These bring the mass of the mouth 
forward again after it has been retracted by the 
broad oblong azygos muscle above described, 
which passes between the pectoral zone and the 
inferior maxilla. 
971 
Head of Lamprey, after Carus, shewing muscles. 
a, b, c, cartilages of the mouth; d,e, f, external 
muscles inserted into ditto; g, h, muscles de- 
rived from the hyoid apparatus. 
In Sharks the lateral muscles of the body 
and fins resemble those of the osseous Fishes. 
Their jaws, however, constructed after the same 
principle as in the Skate, are equally moveable, 
and their muscles almost similar; only here, as 
their mouth is situated much nearer the anterior 
extremity of the skull, the two great muscles 
coming from the spine to the upper jaw are 
wanting. 
Fig. 506. 
- ' Myology of Shark ( Squalus glaucus). After Carus. 
' @, a, a, cranium; }, rostrum ; ¢, olfactory organ; d, eye-ball; e, muscles of eye; St upper-lip ; h, j, 
teeth ; #, lower surface of skull; J, m, muscular masses which close the mouth, resembling those of 
the Skate described above ; g, broad muscle passing from upper to lower jaw; p, depressors of lower 
jaw, asin the Skate; q, 9, g, entrances to the gill-chambers. 
In the Lampreys ( Petromyzonide ) the oral 
sucker is moved by slips derived from the ante- 
rior temination of the great lateral muscle (fig. 
507,f)) as well as by a set of very strong fasciculi 
derived from the hyoid apparatus, which, by 
retracting the interior of the disc, cause the 
adhesion of the sucker, and move the different 
parts of the dental apparatus described in a 
preceding page, (g, 4, m.) The action of 
these will, however, be better understood by 
inspecting the figures than by any detailed 
description. 
Tegumentary system.—The essential character 
of the skin, says Agassiz,t is that it completely 
envelopes an animal, and thus forms a kind of 
external skeleton which protects it over its whole 
* Cuvier, Legons d’Anatomie Comparée. 
+ Agassiz, Recherches sur les Poissons Fossiles, 
4to. 1834, 
surface, as the osseous skeleton protects and 
supports the internal viscera. In the invertebrate 
races of animals there are no other solid parts 
except those which are produced by or con- 
nected with the tegumentary system, but which 
nevertheless can by no means be compared 
with the osseous system of the Vertebrata, 
which is quite peculiar to the latter, and has 
no analogy whatever with the solid framework 
of the inferior classes. 
The skin, moreover, (observes the same 
illustrious author,) is not exclusively limited 
to the external surface of the body, but pene- 
trates into and invests the internal cavities, 
on the inner surfaces of which it likewise pro- 
duces solid structures of various kinds to 
which different offices are assigned, as, for 
example, the teeth and all the corneous pieces 
which in many classes are met with upon the 
lining membrane of the digestive tube. It 
