CLASS FISHES. 289 



control and coordinate the activities of these lower centres. 

 Trace these structures in Fig. 112. 



The fishes have the sense of touch developed in the 

 skin; a chemical sense (similar to taste and smell) that 

 gives some information of the condition of the water, 

 located in the nostrils, and possibly on the snout and in the 

 mouth ; a sense of mechanical jar, and possibly of sound, 

 in the ear and the organs of the " lateral line"; and a kind 

 of gravity or equilibrium sense located in the ear. The 

 eyes are well developed and are similar in general plan and 

 structure to those of higher vertebrates. (Fig. 159.) 



299. Reproduction and Development. The sexes are 

 separate in fishes, as in all the higher vertebrates. The 

 ova and sperm are produced in great numbers. The ova 

 (spawn) are usually deposited in the water in shallow 

 places on the open bottom, under rocks, or in nests pre- 

 pared for them by the parents. The sperm (milt) is 

 poured over these by the male and the fertilization and 

 later development are left to chance. Great numbers 

 of eggs are laid, the most of which perish. In a few cases 

 some care is given to the young by the parents. In some 

 types the eggs are fertilized and the young hatch in the 

 body of the mother. Only a few young are produced in 

 these cases. 



The eggs of fishes have considerable yolk and the 

 young usually hatch out in the adult form. 



The breeding habits of many fishes are exceedingly 

 interesting. The mature salmon live in the sea; but when 

 spawning time comes they migrate in great shoals up the 

 fresh-water streams, leaping all sorts of obstacles in their 

 effort to get to the cool head- waters. Here the eggs are 

 laid and the young pass through the early stages of their 

 19 



