352 



METAZOAN PHYLA 



them ureters carry the excretions to a urinary bladder which opens 

 to the outside through a urinogenital sinus located posterior to the anus. 



The brain consists of small cerebral hemispheres, small olfactory 

 lobes, large optic lobes, a large cerebellum, and a medulla (Fig. 239). 



381. Food of Fishes. — The food of fishes is highly varied, consisting of 

 aquatic vegetation; of all the smaller forms of animal life found in the 

 water, such as insects, crustaceans, mollusks, and worms; and in some 



Brcinchiosfegal 

 membrane 



Gills 



Oral valve 



Mouth openinof 



Upper valve 

 Lips 



Lower valve 



Fig. 240. — Diagrams to illustrate the mode of breathing in teleosts. {From Dahlgren, 

 Zool. Bull., vol. 2.) A, the passage of water into the mouth; B, its passage out through the 

 gills; C, front view of the mouth of a sun fish, Eupoviotis gibbosus (Linnaeus). The anterior 

 part of the mouth cavity is shown in vertical section in .-I and B, the posterior part in 

 horizontal section. The large arrows indicate both the movement of the water and the 

 pressure exerted by it; the smaller arrows the direction of movement of the walls of the 

 mouth. 



cases of larger animals, including not only other fishes but amphibians 

 and higher vertebrates which accidentally get into the water. Some are 

 distinctly predaceous and others more decidedly herbivorous; still others 

 gather mud and debris from the bottom, straining out the living and 

 dead organisms which it contains. All fish are very voracious. The 

 ultimate source of the food of most fishes is the plankton (Sec. 537) which 

 the water contains and which, though not often serving directly as fish 

 food, provides food for the multitude of organisms upon which fish feed. 



