424 COLLEGE ZOOLOGY 



It is widely used for laboratory study, and detailed accounts of 

 its anatomy may be found in several laboratory manuals. It 

 will suffice here to point out certain of its more prominent 

 characteristics. 



External Features. 1 — The body is fusiform and about two 

 and one half feet long. There are two dorsal fins (Fig. 359, D) 

 each with a spine (not shown in Fig. 359) at the anterior end, two 

 pectoral fins, and two ventral fins {VF). The ventral fins in the 

 male possess cartilaginous appendages, known as claspers {CL'). 

 The tail is heterocercal (see Chap. XVII). The mouth is a trans- 

 verse slit on the ventral surface of the head. On either side above 

 the mouth is an eye, and in front an olfactory 

 organ (Fig. 359, N). Anterior to each 

 pectoral fin are six gill-slits (GS), the first 

 of which is situated just back of the eye 

 scale of ° 'Greenland and m °dified as a spiracle {S P) . Between 

 shark viewed from the ventral fins is the cloacal opening (CL). 



side and top. (From rpi <■ • i ..-i . 7 • j ' . 



Dean x v I he surface is covered with placoid scales 



or dermal denticles (Fig. 360) which form 

 shagreen. They represent a primitive exoskeletal structure 

 and have been the starting-point for the development of the 

 scales and bony plates of the true fishes. 



Over the jaws they are modified as teeth with their points 

 directed backward, and are used for holding and tearing prey. 

 A placoid scale consists of a bony basal plate with a spine in the 

 center composed of dentine and covered with enamel. 



The Skeleton. — The skeleton is cartilaginous. The axial 

 skeleton consists of the vertebral column, skull, and visceral 

 arches. The vertebrce (Fig. 359, C) are hour-glass-shaped 

 (amphiccelous), and the notochord persists in the lenticular spaces 

 between them. The skull is much more highly developed than 

 that of the cyclostomes. It is composed principally of the 

 cranium or brain case (CC), two large anterior nasal capsules, 



1 Figure 35Q shows the anatomy of a shark which differs slightly from that of the 

 dogfish shark. 



