432 



VERTEBRATE LIFE AND ORGANIZATION 



Branchial arches 



Jaws: 



mandibular^ 



arch 



Gill slits- 



Figure 22.6. A, The "spiny shark." Climatius, was among the first jawed verte- 

 brates, riiis fish was about three inches long. After removal of the gill covering and 

 superficial bony scales and plates on the head of a related placoderm {Acanthodes, B), 

 it can be seen that the jaws are modified gill arches. {A, After Watson; B, modified 

 after Watson.) 



A well known placoderm is the "spiny shark," Climatius (Fig. 

 22.6 A). It was a streamlined fish which retained the primitive hetero- 

 cercal tail and a covering of thick, bony scales, but the scales were 

 smaller, so that a greater freedom of movement of the trunk would have 

 been possible. Stabilizing paired appendages were present, but instead 

 of a pair of pectoral and pelvic fins there was a long series of paired 

 ventrolateral spines. Each may have supported a web of flesh. Fishes 

 with these features were probably able to move throvigh the water with 

 fair rapidity. 



A major advance was the development of jaws, which evolved as 

 a modification of a gill arch (Fig. 22.6 B). The numerous gill arches of 

 ostracoderms appear to have been firmly united with the bony plates 

 covering the head. In other fishes each gill arch is movable and more 

 or less > -shaped, with the apex of the > hinged and pointing poste- 

 riorly. During the evolution of jaws, certain of the anterior gill arches 

 were lost, but the most anterior remaining arch became enlarged and, 

 together with bone developed in the skin adjacent to it, formed the 

 jaws. This arch is known as the mandibular arch. The arch next pos- 

 terior is the hyoid arch, and the remaining are typical gill or branchial 

 arches. In higher fishes the hyoid and mandibular arch are very close 

 together, the hyoid often helping to support the jaws, and the gill slit 



