A HISTORY OF VERTEBRATES: FISHES 433 



that one might expect to find between these arches is either absent or 

 vestigial. It is of interest that the mandibuhir and hyoid arches of 

 placoderms were not so close together, and there was a complete gill 

 slit between them. Here we actually see an evolutionary stage that we 

 would theoretically expect. The development of jaws was an important 

 step in the evolution of vertebrates, for the presence of jaws enabled 

 vertebrates to adapt to many more modes of life. The success of the 

 jawed vertebrates doubtless led to the extinction of ostracoderms, and to 

 the limitation of cyclostomes to rather specialized ecologic niches. 



198. Characteristics of Cartilaginous Fishes 



Although a few ostracoderms and some of the later placoderms en- 

 tered the sea, most of these early vertebrates were fresh-water animals. 

 The first fishes to achieve lasting success in the ocean were the sharks, 

 skates and their relatives of the class Chondrichthyes. The earliest mem- 

 bers of this class appeared during the Devonian period. They were 

 marine and the group has remained marine except for a few species 

 that have secondarily entered fresh water. 



Although they originated from some early placoderm stock, quite 

 possibly the "spiny sharks," the cartilaginous fishes differ from placo- 

 derms in many ways (Fig. 22.7). In general these fishes are highly stream- 

 lined, yet they retain the primitive heterocercal tail. Only pectoral and 

 pelvic paired fins are present, and these are fan-shaped structures sup- 

 ported internally by a A\ell developed appendicular skeleton rather than 

 simply by anterior spines. In early members of the group each fin had a 

 broad attachment to the body, but in recent sharks the base of the fin 

 is rather narrow. The latter type of fin is more mobile and hence more 

 effective in stabilizing and steering. The medial part of the pelvic fin 

 of the male is modified as a clasper, or copulatory organ. 



A lateral line sensory system is well developed. It consists of a 

 canal in the skin that extends along the side of the tail and trunk, and 

 ramifies over the head. This canal may be an open groove, as in the 

 chimaeras (Fig. 22.7 D), or it may be closed, opening to the surface only 

 through small pores. It contains minute sensory organs that enable the 

 fish to detect low frequency vibrations, movements, and perhaps pres- 

 sure changes in the water. Such a system is found in all fishes but is 

 rather inconspicuous in cyclostomes. 



Shark skin feels like sand paper because of the minute placoid 

 scales embedded in it. These scales are the evolutionary remnant of the 

 extensive covering of thick, dermal scales of primitive fishes. The rest 

 of the skeleton is cartilaginous, not bony. A cartilaginous cranium, ver- 

 tebral column, appendicular skeleton and visceral skeleton are present. 

 Calcium salts may be deposited in the cartilage and may strengthen it, 

 but there is never any ossification. A skeleton coinposed of cartilage is 

 believed to represent the retention in the adult of the embryonic 

 skeletal material. It is not regarded as the primitive adult condition, 

 for these parts of the skeleton were at least partly ossified in earlier 



