972 
therefore becomes necessary to distinguish two 
modifications of the dermal skeleton, one con- 
Stituting the investments of the external sur- 
face of the body, the other developed from 
the internal surface. These two kinds of 
exo-skeleton exist simultaneously in all verte- 
brate animals in addition to the endo-skeleton 
or proper osseous system, which encloses the 
visceral cavities or affords a framework around 
which the soft parts are situated. In the ver- 
tebral division of the animal kingdom not only 
do these two modifications of the dermo- 
skeleton present numerous connexions with 
each other, but they are likewise intimately 
connected with the osseous system, and in 
many parts of the body insensible transitions 
may be perceived between one and the other, 
as in Fishes, more icularly between the 
opercular bones and the scales, or between the 
latter and the bones of the occiput and hu- 
merus, or between the pharyngeal bones and 
the teeth, (See Osszous System, Comp. Anat. 
vol. iii., page 846.) 
There exists, however, a constant antago- 
nism in the developement of the three kinds 
of skeleton above indicated, some of the dif- 
ferent parts of which attain to a more perfect 
growth in proportion as those of the other are less 
complete in the different regions of the body. 
Before proceeding further with this subject, 
it will be necessary to examine with a little 
attention the structure of the skin itself prepa- 
ratory to describing the various dermal appen- 
dages produced therefrom. 
The skin of Fishes is always much more 
tensely stretched over the surface of the body 
than in other animals, and, being closely united 
to the subjacent muscles by dense cellular 
tissue, is never endowed with that mobility 
which is observable among many of the higher 
Vertebrata. As in other classes, the skin is 
composed of an epidermis which forms the 
external envelope of the body, of a rete mu- 
cosum, consisting of the internal stratum of 
the corneous epidermis, which as yet remains 
soft and covers the surface of the corium by 
which it is secreted ; and lastly of the corium 
itself, or the internal living skin furnished with 
nerves and vessels by which the outer layers 
of the integument are secreted as well as the 
different colours that ornament the exterior of 
the tish. 
The varied colours of the fish result in fact 
from the deposition of corresponding pigments 
between the epidermis and the true skin, and 
in the class before us these colouring matters 
are particularly abundant. In the first place 
the inner surface of the scales is imbued with a 
pigment of metallic splendour, and generally 
of a silvery or golden hue and of brilliant lus- 
tre, besides which, more especially towards the 
back and over the re aspect of the body, are 
points or patches of black or diversely coloured 
pigments, which according to their abundance 
and character give the peculiar markings of the 
fish. 
The material which gives this metallic lustre 
to the scales of Fishes, known in commerce 
under the name of “ argentine,” was minutely 
PISCES. 
investigated by Reaumur,* who found that, 
when examined under high magnifying powers, 
it is — of crystalline lamine, divided 
transversely so as to form rectangular figures 
about four times longer than they are broad. 
These crystals he believed to be contained in 
vessels, or in delicate tubes of animal matter, 
mistaking for vessels the little bundles in which 
they are disposed.¢ These different pigments 
have been lately discovered to consistofextremely 
minute crystals of various earthy and metallic 
substances ; they are met with even in the interi- 
or of the body, as for example, upon the external 
surfaces of the peritoneum, of the brain and me- 
dulla oblongata, and in the interior of the e 
ball. Ehrenberg observed them in the Pike; 
they are met with in all Fishes, and present nu- 
merous varieties of form and composition in dif- 
ferent species. One very remarkable phenome- 
_ connected with the colour of a and 
which a ntly depends upon the abundance 
of these piconets sd the sapidity with which 
they are secreted and absorbed, is the ¢ 
of colour which many species undergo at diffe- 
rent periods of the year, as, for example, at 
spawning time; or during their growth, or even 
when excited to violent exertion, or lastly after 
their death, when they are exposed to different 
atmospherical influences. During the spawn- 
ing season, observes Agassiz, the tints of all 
species hitherto observed are more vivid and 
distinctly marked than at other periods; but — 
even whilst drawing living specimens, he has” 
observed that, when suddenly irritated or whilst — 
making violent movements to escape from the 
hand when seized, these colours suddenly be- 
come much deeper and more brilliant, after — 
which they become completely pale, and only 
return by slow degrees; a phenomenon which 
the writer above quoted supposes to depend 
upon a sudden exuberant secretion and subse- 
quent absorption of the coloured pigments. 
The surface of the body of living Fishes is 
moreover constantly lubricated by a great quan= — 
tity of mucus, which in some is possessed of 
little tenacity, and forms a very thin layer 
whilst in other species, especially in such 
have but slightly der Gpad scales, it is of me 
consistency and furnishes a covering of consi- 
derable thickness, as for instance in the ch 
and Eel. This fluid is secreted by a 
us canal, which extends along the whole 
ength of the body and ramifies extensively in 
the bones of the head. The fluid which it se~ 
cretes, which is very viscid and difficult to mn 
with water, exudes at the surface through a 
number of orifices which are visible upon th 
cranium, upon the bones of the face, along the 
jaws, upon the preoperculum, and hkewise 
through the series of tubes which perforate the 
scales along the lateral line. From thes 
sources it is distributed all over the surface of 
ao de l’Academie Francoise, 1716 
+ This substance, argentine, under the name 
** essence de l’orient,” was extensively employe 
in the time of Reaumur for the manufacture of art 
ficial pearls, and was on that account an 
ingly costly article, 
+ ; - 
do 
ye 
