PISCES 113 



istic of plastic or ever-juvenile races. The broad, flat types such as 

 skates, rays and torpedoes, illustrate the bottom-feeding, compara- 

 tively sluggish adaptive complex, but this has not reached its extreme; 

 for though these forms are more heavily armed with spines and small 

 dermal plates than are the sharks, none of them have acquired really 

 heavy armor, nor a definitely sedentary habit. 



A Typical Elasmobhanch 



The dog-fish Sgualus acanthias (Fig. 61) may be taken as a typical 

 elasmobranch in order to introduce the group. These rather small 

 predaceous sharks are called by the natives of our Atlantic coast 

 "Homed Dogs" or "Spiny Dogs" to distinguish them from the sim- 

 ilar but smoother dog-fish, Mustelus, which belongs to a different 

 family. In Buzzards Bay and Vineyard Sound the species was a few 

 years ago so abundant as to be a real pest and they were caught in 

 large quantities and used only for fertihzer. They have recently been 

 discovered to be an excellent food fish and are now being put on the 



Fig. 61. — ^The dogfish shark, Sguahis acanthias. (From Hegner, after Dean.) 



market in canned form under the name of "gray fish." They rove 

 the coastal waters in schools and destroy large numbers of smaller 

 fishes, squids, ctenophores, and many worms. They are caught in 

 fish traps where with their sharp teeth they do a good deal of damage 

 to the mesh. Squalus is viviparous, giving birth to young "pups" 

 upwards of six inches in length. The figures used to illustrate the 

 anatomy of the dog-fish, are for good reasons, not of the species likely 

 to be used in the laboratory, but correspond sufficiently closely with 

 descriptions. 



External Characters (Fig. 61). — The body is submarine-shaped, 

 sharp at both ends. The steering and balancing organs consist of 

 two median dorsal fins, two pairs of lateral fins, pectoral and pelvic, 

 the latter of which are in the male provided with stiff specialized 

 portions known as claspers, used for holding the female during cop- 



