668 EVOLUTION OF ANIMALS Part V 



animals. Their sizes are various. The whale shark grows to be 40 feet long; the 

 pygmy fish (Pandeka) of the Philippines is less than half an inch long. 



All fishes, except the sturgeons and lung fishes, have a more or less well- 

 developed vertebral column. The nervous system has essentially the same 

 arrangement as the frog. The sense organs differ from those of the latter 

 mainly in degree of development. In the skin, fishes have chemical senses 

 similar to taste and smell; they also have organs of smell in the nostrils. The 

 lateral line organs are rows of pits containing cells that are very sensitive to 

 changes in pressure and to any commotion in the water — even a fish passing 

 by. To a large extent fishes can find their way by means of their skins. 



Fishes are classified according to the condition of their skeletons. At one 

 extreme, in general the most primitive, are the sharks and rays, the elas- 

 mobranchs (Class Chondrichthyes), whose endoskeletons are cartilaginous 

 except for whatever beads of notochord still persist (Figs. 33.6, 33.7). At 

 the other extreme is the great group of teleosts or bony fishes (Class Oste- 

 ichthyes), true eels, catfishes, swordfishes, trout, perch, mackerel and scores of 

 others, that are familiar at least in books and the fish market. Their skeletons 

 are the most completely bony of any fishes. Between these two extremes are 

 fishes whose skeletons are partly cartilage and partly bone in various pro- 

 portions. 



Bony Fishes. There are more than 12,000 species of bony fishes, one or 



Fig. 33.6. Upper, dogfish (spiny dogfish or shark) (Squalus acanthias) . A 

 bottom feeder, commonly 2 to 3 feet long. Lower, dogfish shortly before birth. 

 The yolk sac containing the still unused yolk protrudes from the body wall for 

 some time after birth, but becomes gradually smaller. ( Upper, courtesy. General 

 Biological Supply House, Chicago, 111. Lower, courtesy. Rand: The Chordates. 

 Philadelphia, The Blakiston Co., 1950.) 



