246 COMPARATIVE ANATOMY 



of the vertebrae are distinct; the notochord is largely per- 

 sistent ; the jaws are so loosely connected with the cranium 

 that their primitive relations to the skull are readily discernible. 

 The teeth are seen to be nothing more than modifications of 

 skin denticles. The branchial skeleton is well developed, and 

 undergoes none of the modifications which in higher types 

 render its origin obscure. The musculature of the body is 

 relatively simple and retains its primitive segmentation. The 

 excretory organs retain their segmental arrangement to a much 

 greater degree than in the frog. The heart has only a single 

 auricle and a single ventricle, contains only venous blood, and 

 escapes all the complications attendant on aerial respiration. 

 The general plan of the arterial system is simple, and is 

 distinctly correlated to the gills and gill-slits ; we shall see that 

 in the higher vertebrates the arterial arrangement is evolved, 

 in the course of individual development, from just such a 

 system as is persistent in the adult dogfish. The distribution 

 of the yth, the 9th, and the loth cranial nerves affords a clue 

 to the more obscure and complex distribution of these nerves 

 in higher forms. Lastly, the primitive character of the adult 

 dogfish is so far shared by its embryo that we have evidence of 

 the segmentation of the head, of the origin of the eye-muscles, 

 and of the relation of cranial nerves to head segments, not to 

 mention a number of other characters, such as is not to be 

 obtained by the study of any of the higher vertebrates. 



The embryology of the dogfish is, indeed, as interesting and 

 instructive as its adult anatomy, but for various reasons it will 

 not be considered here. For one thing, the ovum is so greatly 

 distended with food-yolk that the segmentation and earlier 

 phases of development are modified to an extent which renders 

 their interpretation difficult, unless the student has already 

 acquired a considerable familiarity with a more normal type 

 of vertebrate embryology. Such a type is furnished by the 

 common frog, and we will therefore consider it in some detail 

 in the next chapter. 



