Whence the Shadows? 217 



cal than real fish. One genus (Callorhinchus), for example, has a trunk- 

 like proboscis or snout, and is variously known as the Elephant shark or 

 the Southern Beauty, depending on one's sense of humor. 



Elasmobranchii takes in both the Selachii, or sharks, and the Batoidei, 

 or skates and rays, which are very close relatives of the sharks. The en- 

 tire family, including all the species of sharks, skates, rays, and oddities 

 in between, are sometimes called the Selachians, and if we accept 

 Selachian as a kind of informal family name, we have one word that 

 covers all of these types. 



There are about 40,000 known species of fish in the waters of the 

 world. They can be roughly divided into three types: the Cyclostomes, 

 eel-like creatures such as lampreys and hagfish, which have no jaws, no 

 bones, and are so primitive that experts do not all agree on whether or 

 not they are true fish; the Teleosts, which have bony skeletons; and 

 the Selachians. 



The overwhelming majority of fishes are Teleosts. However, there 

 are at least 250 species of sharks, and some authorities put the estimate 

 as high as 350. New species are being reported by every important 

 oceanographic investigation. There are 300 to 340 species of skates and 

 rays, and undoubtedly many more remain to be discovered and classi- 

 fied. Numerous as Selachian species may be, their diversity does not 

 compare with that of the Teleosts. Because Teleosts are so numerically 

 superior to the Selachians and, because the Teleosts have bony skele- 

 tons like man's, they are sometimes called "higher fish" or "true fish." 

 Some ichthyologists, in fact, prefer not to class the sharks as true fish at 

 all. But the typical shark is usually rated by all other zoologists as a fish. 



No matter its classification, it is well adapted to the sea. In the fish- 

 eat-fish world beneath the waves, few fish regularly feed upon the shark 

 —except the shark. There are sea creatures that may challenge the shark 

 —Killer whales and an occasional swordfish. Like man, however, the 

 shark is normally prey only to its own kind. 



The basic difference between Teleosts and Selachians is skeletal. 

 Sharks have no bones, only cartilage; the Teleosts have true bones. But 

 there are several other basic diflferences, the technical details of which 

 are beyond the scope of this book. However, these may be summed up 

 as shown in the accompanying table (page 218). 



The digestive system of Selachians is very primitive in structure; 

 the flesh contains urea which gives it a distinctive odor and causes more 

 rapid decomposition than in most Teleosts. The pectoral fins in many 

 species are capable of little or no swimming movement; the breathing 

 organs include not only gill slits but also spiracles on the sides or top of 

 the head. The bodies of most sharks are shaped much like those of some 



