CHAPTER 25 



Class Chondi ichthyes. 



Cartilaginous 



Fishes 



HE Chondrichthyes (cartilage fishes), also 

 called elasmobranchs, are the sharks, rays, 

 and skates. They are the most generalized 

 of the living vertebrates that have com- 

 plete vertebrae, movable jaws, and paired 

 appendages. 



SQUALUS ACANTHIAS- 

 A DOGFISH SHARK 



The common dogfish shark, known since 

 Aristotle's time as being ovoviparous, is 

 abundant in the waters off the coasts of 

 New England and northern Europe. It is 

 of special biologic interest because many of 

 the basic vertebrate features are present in 

 this shark in simple form, and this helps 

 one to understand the more complex sys- 

 tems of higher vertebrates. This is an an- 

 cient group of fishes and is represented by 

 many fossil remains. 



External anatomy 



The body is spindle-shaped and about 

 IVi feet long. There are two dorsal fins, 

 one behind the other, each with a spine at 

 the anterior end, two pectoral fins, and two 

 pelvic fins. The pelvic fins in the male 

 possess cartilaginous appendages known as 

 claspers. The tail (caudal fin) is heterocer- 

 cal (Fig. 260). The mouth is a transverse 

 slit on the ventral surface of the head. On 

 either side, above the mouth is an eye, and 

 each nostril on the ventral side of the head 

 opens into a blind pouch, the olfactory sac. 

 Anterior to each pectoral fin are 6 gill slits, 

 the first of which is situated back of the eye 

 and modified as a spiracle. Between the 

 pelvic fins is the cloacal opening, some- 

 times called an anus. 



The gray-colored skin is covered with 

 placoid scales (Fig. 245). For a dorsal view 



Figure 244. Facing page, the internal organs of 

 the spiny dogfish. The veins are in sohd black, the 

 arteries in outline. 



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