FISHES. 309 



In the osseous fishes, the openings of the mucous ducts 

 are chiefly observable in the fore part of the head, and in 

 the lateral line. This hne extends from the head to the 

 tail, along each side of the fish, and exhibits several strik- 

 ing peculiarities. It is not observable in the lampry ; in 

 general it is single, but in the sand-eel it has the appearance 

 of being double. It is usually of a different colour from 

 that of the sides, and varies according to the species in 

 position and direction. After death it sometimes disap- 

 pears, and hence some difficulties have arisen with regard 

 to the discriminating marks which it furnishes. 



The mucus which is poured out upon the skin by these 

 ducts, in some cases appears to be the liquid known by 

 chemists under that name, while in other instances it ap- 

 pears to be of the nature of albumen. In the eel, for ex- 

 ample, the skin turns white from the coagulation of tlie 

 albumen, when plunged into boiling water. Chemists, how- 

 ever, have not turned their attention to the subject. 



The JBoNEs of fishes vary in form, proportion, and num- 

 ber, according to the specie. The skeleton is more com- 

 plicated than that of man, and is difficult to prepare and 

 preserve. Hence the osteology of fishes is a sviliject but 

 little attended to by naturalists. Avoiding all minute de- 

 tails, we propose to consider the skeleton, as consisting of 

 a head, spine, and ribs. 



As the head of fishes is covered with a skin only, its 

 form is easily ascertained, and it exhibits remarkable differ- 

 ences in shape according to the species In all the species 

 it is large in proportion to the size of the body, and con- 

 sists of a great number of separate pieces. These amount 

 to eighty in the perch. But as these bones are soon ossified 

 together, it becomes very difficult to trace the original lines 

 of separation in aged individuals. The occiput appears 

 like a vertical truncation of the cranium, and is united .to 



