240 BIOLOGY FOR BEGINNERS 



The mouth is usually at the extreme anterior since it is the only 

 organ for food-getting or defense, and it is provided with numerous 

 sharp teeth, arranged on three sets of jaw bones and slanting in- 

 ward so that there is little chance for a victim to escape. 



FIG. 84. Fish External Features. 



The Fins can be divided into those on the median line and those which are 

 paired. The former are probably parts of a continuous fin which, in earlier 

 forms, extended completely around the body, as in the eel or tadpole. 



The dorsal fins can be erected and are armed with spines for protection. 

 Smaller spines are also found in the anal and pelvic fins. 



The caudal fin is the chief propelling organ and has flexible fin-rays for its 

 support. All the fins in the median line aid in locomotion and steering. 



The paired fins are homologous to the limbs of higher animals. The pelvic 

 fins aid in supporting the fish when at rest on the bottom, and both pairs help 

 in balancing and swimming. 



The Lateral Line seems to consist of a series of gland-like sacs whose function 

 is thought to be to provide a depth or pressure sense. 



The Nostrils have two openings each, so that water can flow through them 

 as the fish swims, bringing with it the particles which cause the sensation of 

 smell. They do not connect with the throat and have nothing to do with 

 breathing, as is the case of air breathers. 



The Scales are arranged overlapping to the rear, to give all possible protec- 

 tion, and at the same time permit perfect freedom of motion, and offer no 

 resistance to the water. A slippery secretion aids in locomotion and escape 

 .'rom enemies. Often their color is of advantage in escaping observation, either 

 by enemies or prospective prey. 



The Operculum is a strong covering which protects the very delicate gills 

 from injury. It has a slight motion, so as to permit the water to pass out under- 



