CLASS OF FISHES. 



driven across the gills and escapes by these openings. Gene- 

 rally there is but one on each side, and it has a moveable 

 protecting covering; finally, there is throughout the whole 

 length of the body on either side a series of pores, called the 

 lateral line. The skin is 

 sometimes almost naked, 

 but in general is covered 

 with scales ; these have oc- 

 casionally the form of rude 

 grains, sometimes of very 

 large tubercles or plates; 

 but generally they resemble 

 very thin lamellae, over- 

 lapping each other like the 

 tiles of a house, and en- 

 cased in folds of the skin. 

 They may be compared to 

 our nails, but generally 

 they include much more of 

 the calcareous salts. The 

 colours of fishes astonish by 

 their variety and brilliancy, 

 resembling gold and silver : 

 these depend on a number 

 of small polished plates se- 

 creted by the skin. 



476. The skeleton of 

 fishes is either osseous or 

 cartilaginous: in the lam- 

 prey it remains almost 

 membranous, and thus esta- 

 blishes a link between the 

 vertebrata and invertebrata. 



477. Their bones have 

 no medullary canal, and 

 when boiled in water they 

 give out no gelatine. The 

 skeleton is composed of a Fig. 307. Skeleton of the Perch, 

 head, trunk, and limbs, 



with a hyoid apparatus largely developed, and assisting in 

 respiration. 



478. The structure of the head is very complex, and 

 composed of many bones ; its median portion (Fig. 308) 

 z 



