668 



ICHTHYOLOGY. 



Structure 



nd 



I'mu'tinns 

 nf Fishes. 



ting in quantity. In one division of fishes, termed the 

 cartilaginous, the proportion of earthy matter is so small, 

 that the bones never become indurated, but continue 

 in all the periods of the life of the fish soft and flexible. 

 These animals are therefore supposed to grow during 

 the whole course of their existence. 



When the bones of some fishes are boiled in water, 

 they undergo a change of colour. This circumstance 

 is well illustrated in the case of the gar-fish, or sea pike, 

 (Esox bclone,) whose bones by boiling become of a grass 

 green colour; and in the bones of the viviparous blenny, 

 which experience a similar change. This alteration of 

 colour has fostered some of the prejudices of the vulgar, 

 but has failed to arrest the attention of the chemist. 



The bones of fishes when reduced to powder, are 

 mixed up with farinaceous substances, and used instead 

 of bread by some of the northern nations. In Norway, 

 and even in some of the remote districts of our own 

 country, fish bones are given as food for cows, and are 

 greedily devoured by them. 



SECT. II. Organs of Protection. 



Organs of UNDER this head, it is our intention to consider the 

 protection, skin, the scales, and the spines of fishes. The skin of 

 fishes consists, as in the other vertebral animals, of a 

 true skin, a rete mucosum, and a cuticle. The cutis, 

 or true skin of fishes, is remarkably thick in those spe- 

 cies which have small scales ; while in those which 

 have large scales, it frequently assumes the appearance 

 of a thin membrane. It is much more closely attached 

 to the muscles in this tribe than in any of the other ver- 

 tebral animals. This organ in the gadi, for example, 

 consists almost entirely of gelatine, and hence is much 

 esteemed as an article of food, and is used also in fining, 

 as a substitute for isinglass. Eel skins are likewise 

 used in the manufacture of size, in consequence of the 

 gelatine which they contain. 



In the higher classes of vertebral animals, there is an 

 organ termed the corpus papillare, or the villous sur- 

 face of the skin, in which the sense of touch is supposed 

 more particularly to reside. Fishes, however, are des- 

 titute of this organ ; and hence anatomists have con- 

 cluded, that these animals are possessed of this sense 

 in a very imperfect degree. 



Intermediate between the true skin and the cuticle, 

 is situated the rele mucosum. It consists of a mucous 

 layer, in which the colouring matter of the skin resides. 

 In the animals which we are now considering, this layer 

 is remarkable for the brilliant tints which it exhibits, 

 communicating to the scales all their metallic lustre. 



The cuticle, or external layer of the skin, appears in 

 fishes in a soft state, and, in many instances, is a simple 

 mucous substance enveloping the body. It is detached 

 at certain seasons of the year in large pieces. 



The scales are implanted in the cuticle, and in their 

 position and use resemble the hairs on the bodies of 

 quadrupeds. They cover the body of fishes like tiles 

 on the roof of a house, pointing backwards. The pos- 

 terior edge, which in general is free, is usually crescent- 

 shaped, fringed in some species, and smooth in others. 

 By means of a lens, longitudinal ribs may be perceived 

 finely decussated by transverse striae. These ribs some- 

 times radiate from the centre, and the crossing strise are 

 concentric. When macerated in weak acids, they are 

 found to consist of alternate layers of membrane and 

 phosphate of lime, and hence are supposed to increase in 

 every direction by the addition of new layers. 



Instead of imbricated scales, some fishes are protect- 



ed by osseous plates, covered, like the scales, by the cu- 

 ticle, and presenting an even surface. Among some of 

 the sharks, as the Squalus acanlliias, instead of scales 

 there are flat bent bristly lamina?; and in the remora 

 there are hard rough tubercles. These osseous plates 

 in the sturgeon, resemble in shape the shell of a limpet. 



These scales may be considered as the ordinary ar- 

 mature of fishes. They guard their bodies from ex- 

 ternal injury, and, when rubbed off' by accident, they 

 are reproduced. 



The naturalist employs the appearances exhibited by 

 the scales, as a character in the discrimination of nearly 

 allied species. The form, the surface, and the size of 

 the scales, are chiefly used for this purpose, although 

 the disposition of the longitudinal and the transverse 

 rays, together with the condition of the margin, would 

 furnish more permanent marks. The scales in the de- 

 scription of a fish, are likewise considered in regard to 

 their adhesion to the skin. Thus some scales, which 

 adhere but slightly, are said to be deciduous ; while 

 others, which cannot be rubbed off but with difficulty, 

 are termed tenacious or adhesive. 



Besides the scales, many fishes are furnished with 

 spinous processes. These sometimes accompany the 

 fins ; while in other instances they appear as the arma- 

 ture of the head and cheeks. They appear to be of the 

 same consistence and composition as horn. Those found 

 on the head are in general fixed ; but those connected 

 with the fins are moved by peculiar muscles. These 

 organs may be considered as defensive weapons, and 

 act, in some instances, not merely by their form and 

 consistence, but by some venomous secretion by which 

 they are covered. Thus the common weever ( Trachi- 

 nus draco) inflicts a wound with the spines of the first 

 dorsal fin, often followed by violent burning pains, in- 

 flammation, and swellings ; so that the fishermen are 

 in the practice of cutting off the offensive organ before 

 they bring the fish to market. The spines of the Squalus 

 acanthias, or piked dog-fish, is likewise considered by 

 fishermen as capable of inflicting a dangerous wound. 



The fishes furnished with spinous rays in the fins, 

 were, at a very early period, separated from those with 

 soft rays. They were termed Acanthoplerygii by Arte- 

 di. The fishes furnished with spines on the head or 

 cheeks, have been subdivided by La Cepede into several 

 genera, from the characters furnished by these organs. 



SECT. III. Organs of Sensation. 



IN attending to the organs of sensation in fishes, the Organs of 

 condition of the brain demands our first consideration, sensation. 

 We have already stated, that the head is large in pro- 

 portion to the size of the body ; but with regard to the 

 brain, the reverse of this appears to be the case. It 

 does not completely fill the cavity of the cranium des- 

 tined for its reception, the surrounding space being oc- 

 cupied by a salt fluid. It bears a much smaller pro- 

 portion to the size of the body, than we find in the 

 higher dasses of animals. The following Table of these 

 proportions in a few fishes is given by Cuvier. 



White shark ( Squalus carcharias) 

 Great dog-fish (Squalus caniculd) 



Tunny 



Pike 



Carp 



Silurus glanis 



The brain of fishes is of a less compact texture than 

 that of the superior animals, and in some species is al- 



TT^TW 



