VERTEBRATA: FISHES. 



499 



The Dog-fishes are of common occurrence in British seas, but are of little 

 value. Their egg-cases are frequently cast up on our shores, and are 

 familiarly known as "Mermaid's purses." The embryo possesses external 

 branchiae, developed both from the spiracle and the branchial arches ; but 

 these structures disappear in the course of growth. The true Sharks are 

 not infrequently found in various European seas, but they are mostly 

 inhabitants of warmer waters. One of the largest is the " White Shark." 

 (Carcharias vnlgaris}, which attains a length of over thirty feet. The 

 body in the Sharks (Squalidce) is not rhomboidal, but is elongated ; the 

 nostrils are placed on the under side of the snout, and the teeth are ar- 

 ranged in several rows, and are in the form of compressed cones. During 

 life, the cartilaginous jaws are so far flexible that their margins can be 

 partially everted, thus bringing more than one row of teeth into use at one 

 time ; but the innermost rows are principally employed to replace the . 

 outer rows, when the latter are worn out. 



c. Batides. This group includes the Rays and Skates, and is distin- 

 guished 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 members of the group the body is flattened out so 

 as to form a kind of rhomboidal disc (fig. 271), the greater part of which 

 is made up of the enormously- 

 developed pectoral fins. The 

 pectoral fins are united by cartil- 

 age with the skull, and there is 

 no median facet upon the occiput 

 for articulation with the first ver- 

 tebra. Upon the upper surface 

 of the disc are the eyes and spir- 

 acles ; upon the lower surface are 

 the nostrils, mouth, and branchial 

 apertures. The flattened bodies 

 of the Rays, however, must be 

 carefully distinguished from those 

 of the Flat-fishes (Pleuronectidce}. 

 In the former the flat surfaces 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 often 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 clavata} 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 ma- 

 chine has been demonstrated by many careful experiments. The Torpedo 

 owes its remarkable power to two special organs the " electrical organs," 

 which consist of two masses placed on each side of the head, and consisting 



Fig. 271. Batides. Raia marginata, one of 

 the Skates. Reduced one - sixth. (After 



271. Batides. 

 e Ska 

 Gosse.) 



