CLASS OSTEICHTHYES. BONY FISHES 



385 



vibrations in water and pressure stimuli. 

 There are also numerous other cutaneous 

 sense organs. 



The two olfactory sacs lie in the anterior 

 part of the skull and communicate to the 

 outside by a pair of openings in front of each 

 eye. They are not connected with the mouth 

 cavity and take no part in respiration. The 

 inner surface is thrown up into folds which 

 contain many sense cells. Ability to detect 

 odors lies in the olfactory sacs. 



The ear consists of the membranous 

 labyrinth only. As in the cyclostome and 

 shark, the sound waves are transmitted by 

 the bones of the skull to the fluid within the 

 labyrinth. Three semicircular canals (Fig. 

 396) are present, and the sacculus contains 

 concretions of calcium carbonate called ear 

 stones or otoliths. Experiments indicate 

 that goldfishes can hear. The ear serves 

 both as an organ of hearing and an organ 

 of equilibrium. 



The eye of the perch differs in several 

 respects from that of the terrestrial verte- 

 brates. The eyelids are absent in bony fishes 

 since the water keeps the eyeball moist and 

 free from foreign objects. The cornea is flat- 

 tened and of about the same refractive 

 power as the water. The lens is almost spher- 

 ical. 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 clearly at about 15 

 inches, but it can detect the movement of 

 objects much farther away. To focus on dis- 

 tant objects the lens is puhed backward. 

 Many fishes are nearsighted. However, the 

 sharks that pursue rapidly moving prey have 

 lenses that are set for distant vision. Recent 

 evidence makes it appear that fishes can 

 distinguish colors; therefore, it is possible 

 that gaudy colors on hook lures are an aid 

 to success in fishing. 



Reproductive system 



The sexes are separate. The single ovary 

 is probably the result of a fusion of two 



ovaries in the embryo. The ovary or testes 

 lie in the body cavity. The germ cells pass 

 through the reproductive ducts and out of 

 the urogenital opening. Perch migrate in the 

 spring from the deep waters of lakes and 

 ponds, where they have spent the winter, to 

 the shallow waters near shore. The female 

 lays many thousands of eggs in a long rib- 

 bonlike mass. The male fertilizes the eggs by 

 depositing sperm (milt) over them. Very 

 few eggs develop because of the numerous 

 animals such as other fishes and aquatic 

 birds which feed on them. 



Development 



The cmbryogeny of the goldfish is substi- 

 tuted here for the perch because it is better 

 known, yet the development of the perch is 

 similar. The young goldfish hatches from the 

 egg in about 3 to 14 days, depending upon 

 the temperature of the water. The egg passes 

 through the stages shown in Fig. 258. A large 

 part of the egg consists of yolk. A proto- 

 plasmic accumulation which forms a slight 

 projection at one end is called the germinal 

 disk ( blastodisk ) . Cleavage of the germinal 

 disk takes place, and the blastoderm pro- 

 duced gradually grows around the yolk. The 

 embryo appears as a thickening of the edge 

 of the blastoderm; this grows in size at the 

 expense of the yolk. After a tjme the head 

 and tail become free from the yolk, and the 

 young fish breaks out of the egg membranes. 

 The young fish lives at first upon the yolk 

 in the yolk sac, but it is soon able to obtain 

 food from the water. 



OTHER FISHES 



External features 



Form of the body 



The bodies of the majority of fishes are 

 spindle-shaped and laterally compressed as in 

 the perch— a form that offers slight resist- 

 ance to progress through the water. Varia- 



