360 METAZOAN PHYLA 



sighted. They can see clearly enough, however, to catch very active 

 prey, and some are even known to capture insects above the water. 

 Others, like trout, evidently detect movement very quickly even at a 

 distance of many feet from the water in which they are. Anableps, a 

 surface minnow, has its eye divided horizontally into two parts (Fig. 254), 

 one part for seeing below the surface of the water and the other for 

 seeing above it. It feeds on insects flying over the water and is found 

 in the streams of tropical America. 



The ear consists of a membranous labyrinth which is lodged in a 

 cavity in the side wall of the skull. This cavity does not form a bony 

 labyrinth. Sound waves to be perceived must be transmitted through 

 the outer wall of the skull and through the tissues forming the wall of the 

 body outside it. In all fishes the ear is mainly an organ of equihbrium. 



386. Behavior.— Fishes, generally speaking, lead very active lives; 

 the predatory ones, especially, are constantly on the alert for food. 

 In our lakes and ponds the smaller fishes retreat to deeper water and 





J'' Sii; 





Fig. 254. — Anableps dovii Gill, a Tropical American fresh-water surface fish, the eyea 

 of which are divided by a partition into two parts, one for seeing outside of the water, the 

 other in it. X M- (From Jordan, "Guide to the Study of Fishes," by the courtesy of 

 D. Appleton & Company.) 



to the protecting cover of vegetation during the day, but at night they 

 approach both the surface and the shore in search of food. This is 

 accompanied by a similar movement on the part of the larger predatory 

 fishes which prey upon them. In captivity, and when given an abun- 

 dance of food, fishes may at times be seen to rest, buoyed up by the water 

 or lying against the bottom, and it is probable that in a natural state 

 they spend brief intervals at ease. Bottom feeders and herbivorous 

 forms go more quietly at the task of securing food than do the predatory 

 types, which dart rapidly at anything that appears like prey. This 

 instinct to snap at a moving object is the cause of the readiness with 

 which predatory fishes take an artificial bait. Some fish, known as gobies, 

 live in holes in the mud of beaches, where they may be dug out at low 

 tide; others are able to attach themselves to rocks by a sucker, or they 

 may hide in cracks and crevices. Some show a definite preference for 

 a certain locality, and a predaceous fish may regularly frequent a 

 particular station from which it watches for prey. Nevertheless fish 

 migrate to avoid adverse conditions such as seasonal changes, to 

 search for food, or to find proper conditions for reproduction. Practically 



