398 



MANUAL OF ZOOLOGY. 



gated ; the nostrils are placed on the under side of the snout, 

 and the teeth are arranged in several rows, of which the outer- 

 most alone is employed,'the inner ones serving to replace the 

 former when worn out. 



c. Batides. — This group includes the Rays and Skates, and 

 is distinguished by the fact that the branchial apertures are 

 placed on the under surface of the body, forming two rows of 



openings a little behind the 

 mouth. In the typical mem- 

 bers of the group, the body 

 is flattened out so as to form 

 a kind of rhomboidal disc 

 (fig. 151), the greater part of 

 which is made up of the 

 enormously-developed pec- 

 toral fins. Upon the upper 

 surface of the disc are the 

 eyes and spiracles ; upon 

 the lower surface are the 

 nostrils, mouth, and bran- 

 chial apertures. The flat- 

 tened bodies of the Rays, 

 however, must be carefully 

 distinguished from those of 

 the Flat - fishes {Pleuronec- 

 tidcs). In the former, the 

 Raia. margitutta, one of flat surfaccs of the body are 

 ^^^" truly the dorsal and ventral 

 surfaces. In the latter, as 

 before remarked, the body is flattened, not from above down- 

 wards, but from side to side, and the head is so twisted that 

 both eyes are brought to one side of the body. The tail in 

 the Rays is long and slender, usually armed with spines, and 

 generally with two or three fins (the homologues of the dorsal 

 fins). The mouth is paved with flat teeth of a more or less 

 rhomboidal shape. 



The typical members of the Batides are the Skates and 

 Rays, of which the common Thornback (Raia davata) may 

 be taken as a familiar example. More remarkable than the 

 common Rays is the Electric Ray or Torpedo, which has the 

 power of discharging electrical shocks, if irritated. The identity 

 of the force produced in this way with the electricity of the 

 machine has" been demonstrated by many careful experiments. 

 The Torpedo owes its remarka'jle powers to two special organs 

 — the " electrical organs," which consist of two masses placed on 



Fig. 151. 

 the 



Batide 

 Skates. Keduced one ■ 

 Gosse.) 



sixth. 



