342 



METAZOAN PHYLA 



and also more primitive than any other known fishes, either hving or 

 extinct. They had a terminal mouth instead of a ventral one. The 

 eggs were probably fertilized externally, since the male had no claspers. 

 In them also the notochord persisted as a continuous rod, the vertebral 

 column being little more developed than that of the cyclostomes. Their 

 characteristics seem to point to a primitive fish ancestor which had an 



Spjrac/e 



trr,.- * >. * >' •*-0». . .■ -■ .. . v VX. 



' ,^. , '/. • • „<» -.i|»:''' i». .' ■■.:■■■*•,■■»•.•'' .■■--/ 





Fig. 231. — Common skate, Raja erinacea Mitchill. X \i,. {Redrawn from Jordan 

 "Guide to the Study of Fishes " by the courtesy of D. Appleton & Company.) 



elongated spindle-shaped body, a mouth armed with dermal teeth, 

 a greater number of gill slits than in any existing forms, paired fins, 

 and a diphycercal tail fin made up of a single lobe. 



366. Economic Facts. — Sharks have always been feared by man and 

 undoubtedly at times they will attack human beings in the water. They 

 are, however, less likely to do so than is generally supposed. One of 

 the dogfish sharks found on the Atlantic coast is an important enemy 

 of the lobster, and a second devours large numbers of valuable food 

 fish. It is stated that the spined dogfish shark, Squalus acanthias 



