that, on account of the embryological facts, the branchial 

 arches of the two groups were not homologous. Holmgren 

 (1942-1946) showed that internal as well as external arches 

 may exist in the embryo of the hagfish. If such is the case, 

 then a part of the branchial skeleton is homologous with 

 that of the gnathostome. 



In view of these opinions, it can be assumed that the com- 

 mon ancestor of both agnaths and gnathostomes had an 

 elementary head skeleton from which both types developed 

 through modification of existing parts, additions, and dele- 

 tions. It may be that the common ancestor had not yet 

 developed a cartilaginous branchial skeleton, and that 

 these two types represent divergences resulting from different 

 functional needs. It seems quite unlikely that the gnatho- 

 stome style was derived from the known agnath, or vice 

 versa. 



RESUME OF THE HEAD SKELETON 



In summary, the head skeleton is an important guide to 

 the understanding of the major subdivisions of the verte- 



brates. On the basis of this area of structure, the agnath can 

 be separated from the gnathostome. These appear to be two 

 independently developed types of head skeletons; neither 

 was probably derived from the other. The gnathostome 

 type can be broken down into subdivisions in each of which 

 a skeletal system was developing at the time of its origin. 

 The endocranium was already partly formed in the stem 

 type so that there are only minor variations of a basic theme 

 in the several lines. Dermal covers for the head appeared 

 independently in each line. One of these, the osteichthian, 

 early in its dermal shield development, gave rise to the acti- 

 nopterygians, choanate fishes, and amphibians. This stem 

 type was close to the ancestral acanthodian and arthrodire. 

 The tetrapod plan is largely a derivative of the osteichthian 

 line. The tetrapod stem gave rise to three modified patterns: 

 mammal, reptile, and living amphibian, each of which shows 

 reduction in the number of bones along with other speciali- 

 zations. The systematic position of the stem tetrapod cannot 

 be based on similarity to any of the three living derivatives. 

 The stem type has arbitrarily been called amphibian simply 

 because of its way of life, laying eggs in water and having a 

 larval stage. 



140 • THE HEAD SKELETON OF FISHES 



