972 



PISCES. 



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 particularly 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 OSSEOUS 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 fish. 



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 upper 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 



investigated by Reaumur,* who found that, 

 when examined under high magnifying powers, 

 it is composed of crystalline laminae, 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. f These different pigments 

 ha ve been lately discovered to consist of extremely 

 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 eye- 

 ball. Ehrenberg observed them in the Pike ; but 

 they are met with in all Fishes, and present nu- 

 merous varieties of form and composition indif- 

 ferent species. One very remarkable phenome- 

 non connected with the colour of Fishes, and 

 which apparently depends upon the abundance 

 of these pigments, and the rapidity with which 

 they are secreted and absorbed, is the change 

 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 as 

 have but slightly developed scales, it is of more 

 consistency and furnishes a covering of consi- 

 derable thickness, as for instance in the Tench 

 and Eel. This fluid is secreted by a muci- 

 parous canal, which extends along the whole 

 length 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 mix 

 with water, exudes at the surface through a 

 number of orifices which are visible upon the 

 cranium, upon the bones of the face, along the 

 jaws, upon the preoperculum, and likewise 

 through the series of tubes which perforate the 

 scales along the lateral line. From these 

 sources it is distributed all over the surface of 



* Histoire de 1'Academie Francoise, 1716 and 

 1718. 



t This substance, argentine, under the' name of 

 " essence de 1'orient," was extensively employed 

 in the time of Reaumur for the manufacture of arti- 

 ficial pearls, and was on that account an exceed- 

 ingly costly article. 



