PISCES 123 



never does. Instead, it feeds only upon small pelagic animals, in- 

 cluding fishes, squids, and other small forms, which it strains out of 

 the water by means of the fringes on its long, slender gill-rakers. 



The Thresher Shark (Fig. 67, C) is remarkable chiefly for the 

 great length of the upper lobe of the caudal fin, which equals the rest 

 of the body in length. 



Angel-Sharks (Rhinidce) constitute an interesting transition be- 

 tween the Selachii and the Batoidei (skates, rays, etc), in that they 

 have a short broad form (Fig. 67, E) with marked lateral expansions 

 of the pectoral and pelvic fins that look like wings and give the group 

 its name. In habits they are more like the rays than the sharks, in 

 that they frequent the bottom and do not follow the free^roving hfe 

 of the typical sharks. 



Sub-order 2. Batoidei (skates, rays, torpedoes), are all, with the single 

 exception of the Saw-Fishes (PristidoB), rhombic or discoidal in form, 

 due to the dorso-ventral flattening of the body and the excessive 

 growth of the pectoral fins. For the most part they are sluggish 

 bottom-feeders that swim slowly over the sea-bottom at various 

 depths, using the pectoral fins as propellers, waves of propulsion 

 passing from in front backward. They are protectively colored on 

 top so as closely to resemble the sea-bottom, but are usually white or 

 uniformly pinkish below. The tail is weak and of little use in locomo- 

 tion, being used merely as a rudder or, in the sting rays, as a weapon of 

 defense. The typical members of the Batoidei are the common skates 

 (Fig. 68, A). These fishes have an almost perfect rhomboidal outline, 

 resembling a broad kite with a short tail. They catch their prey 

 (fishes, crustaceans, etc.) by dropping suddenly upon it and covering 

 it with the broad body and fins. The food is ingested by means of the 

 ventrally placed mouth armed with rasping file-like teeth. Some of 

 the largest species reach a diameter of as much as seven or eight feet. 



The Electric Rays (Torpedinidce) are more nearly circular in 

 body outline (Fig. 68, C) than the skates. They are especially note- 

 worthy on account of the presence of paired electric organs, developed 

 from two pillars of muscle situated between the pectoral fins. They 

 are capable to giving at will quite a heavy electric shock. This mode 

 of defense is in accord with the entire absence of dermal spines, for 

 a fish capable of giving a shock needs no armor. 



Sting Rays or Whip-Tailed Rays (Fig. 68, D).— These tropical 

 Rays are especially noted for the long flexible tail armed with one or 



