l82 



CHORDATE ANATOMY 



in elasmobranchs, other morphologists regard the biserial fin skeleton of 

 Dipnoi as the more primitive. Conclusions in regard to the evolution 

 of the skeleton of the extremity differ, therefore, as one or other of these 

 two types of fish-fin skeleton is assumed as more primitive. 



Summary of Skeletal Evolution. Most animal phyla, even the 

 Protozoa, have some sort of skeletal structures. But there seems to be no 

 genetic connexion between the skeletons of invertebrates and those of 

 vertebrates. In the evolution of a skeleton, vertebrates have been given 



RADIAUA 



A. ICHTHYOPTERYGIUM 



PHALANGES 

 l\ 



n METTACARPALS , 



/ I ICMETATARSALS5 



"^ Ji RADIUS 



^ffl!»->. MS>> |C TIBIA 



5Gt^<g^ , ULNA 



BCHtlROPTERYGIUM. 



Fig. 172. — Diagrams of the pectoral appendages of lower and higher vertebrates. 

 In B. names of corresponding parts of the pelvic appendage are shown in parentheses. 

 A persistent problem of morphology is how the fish extremity or ichthyopterygium was 

 transformed into the fingered extremity (cheiropterygium) of land vertebrates. 



carte blanche. In the process of acquiring a skeleton, chordates first 

 converted the roof of their ahmentary canal into a supporting rod, and 

 later used this as a foundation upon which to build a vertebral column 

 of cartilage. The replacement of cartilage by bone in higher vertebrates 

 is an astonishing engineering feat for which our current theories of adapta- 

 tion seem quite inadequate. 



To the notochord, cyclostomes added neural arches and the rough 

 beginnings of brain case and visceral skeleton. Elasmobranchs show a 

 marked advance towards a more elaborate skeleton. To the vertebrae 



