LUNG-FISH 



379 



branchs, there is no segmenting up of the cartilage to form centra : it 

 remains continuous, except in a short portion near its posterior cml. 



Again the protocercal tail is supported by a skeleton of a very primitive 

 type consisting of parallel rays of cartilage, each segmented into three 

 pieces, which are simply elongated neural and haemal spines. In the 

 other groups of fishes in which the tail has attained to a higher grade 

 of evolution the supporting rays have become crowded together and 

 modified so that at least on the dorsal side their relation to the vertebral 

 arches is no longer apparent. 



The paired fins or limbs again show a type of skeleton which there is 

 much reason to believe is the most archaic and most nearly primitive 



FIG. 164. 



Skeleton of the pectoral fin of various fishes. A, Ceratodus ; B, Plauracanthus ; C, embryo 

 Dogfish ; D, Acanthias ; E, Fossil shark (Cladoselache) ; F, Polypterus larva ; G, Polypterus. The 

 outer or pre-axial side of the limb is to the left, except in A. 



type of limb skeleton existing in any living vertebrate. The skeleton 

 (Fig. 164, A) consists of a segmented tapering rod of cartilage, attached 

 to the limb girdle by its base, and bearing on each side a series of 

 segmented cartilaginous rays. The main reasons for regarding this 

 biserial arehipterygium, as it has been called, as the most nearly primitive 

 of existing types of limb skeleton are the following: (i) It occurs in 

 the paddle of Ceratodus a relatively clumsy, inefficient type of limb, 

 obviously in a very low grade of evolution as a propelling organ compared 

 with the thin flat fins, with muscles concentrated towards the base, found 

 in the other fishes. (2) As we trace the main groups of fishes Elasmo- 

 branchii, Teleostomi and Dipnoi back in geological history we find that 

 in early ages this was the predominant type of limb in all three groups. 

 Consequently we are forced to conclude either (a) that this type of limb 



