CLASS PISCES 441 



The eye of the perch differs in several respects from that of 

 terrestrial vertebrates. The eyelids are usually absent in fishes, 

 since the water keeps the eyeball moist and free from foreign 

 objects. The cornea is flattened and of about the same 

 refractive power as the water. The lens is almost spherical. 

 The pupil is usually larger than that of other vertebrates and 

 allows the entrance of more light rays ; this is necessary, 

 since semi-darkness prevails at moderate depths. When 

 at rest the eye focuses at about fifteen inches. To focus 

 on distant objects the lens is moved back. Fishes cannot see 

 in air. 



The Reproductive System. — The sexes are separate. The 

 ovaries or testes lie in the body-cavity. The germ-cells pass 

 through the reproductive ducts and out of the urinogenital 

 opening. Perch migrate in the spring from the deep waters 

 of lakes and ponds, where they spend the winter, to the 

 shallow waters near shore. The female lays about a hundred 

 thousand eggs in a long ribbon-like mass. The male fertilizer 

 the eggs by depositing spermatozoa (milt) over them. Very 

 few of the eggs develop because of the numerous animals, 

 such as other fishes and aquatic birds, which feed upon 

 them. 



Development. — The young perch hatches from the egg in 

 from two to four weeks, depending upon the temperature of the 

 water. The egg passes through stages similar to those shown 

 in Figure 374. A large part of the egg consists of yolk. A pro- 

 toplasmic accumulation which forms a slight projection at one 

 end is called the germinal disc. The fusion nucleus, resulting 

 from the union of the egg nucleus and the nucleus brought into 

 the egg by the spermatozoon, soon divides, and two cells are 

 formed. Cleavage of the germinal disc continues (Fig. 374, 

 A , B) and the blastoderm (bl) produced gradually grows around 

 the yolk (C-G). The embryo (E, emb) appears as a thickening 

 of the edge of the blastoderm. This grows in size (F, emb, G) 

 at the expense of the yolk. After a time the head and tail be- 



