372 



ZOOLOGY. 



according as they -are placed on the back, under the tail, or at 

 its extremity. They resemble each other in structure, and 

 consist almost always of a fold of skin, supported by osseous 

 or cartilaginous rays, pretty much in the wa}' that the wings 

 of bats and dragons are supported by the fingers and ribs. 



The large openings leading to the gills are placed behind 

 the head; in breathing, the water passing into the mouth is 

 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 occasionally the 



form of rude grains, sometimes of very large tubercles or 

 plates ; but generally they resemble very thin lamella, over- 

 lapping each other like the tiles of a house, and encased 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 secreted by the skirR 



476. The skeleton of fishes is either osseous or cartila- 

 ginous : in the lamprey it remains almost membranous, and 

 thus establishes a link between the vertebrata and inverte- 

 brata. 



477. Their bones have no medullary canal, and when 

 boiled in water they give out no gelatjne. The skeleton is 



* The Barbed Mullet (Mullus Barbatus), to show the different fins, &c. : 

 p, pectoral fin; v, ventral fin; dl, first dorsal; d2, second dorsal; <?, 

 caudal; a, anal; o, opening leading to the gills; b, feelers, or barbs of the 

 lower jaw. 



