240 FISHES 



ventral rays, the original terminal part becoming more bent 

 upwards and reduced, the ventral lobe ultimately forms nearly 

 the whole of the tail, is directed posteriorly, and acquires its 

 new symmetry. On the other hand, in the cod family the tail 

 is really and not merely apparently symmetrical ; it consists of 

 equal numbers of upper or dorsal and lower or ventral rays, 

 and the axis of the vertebral column is continued through the 

 centre of the tail, not bent up to the dorsal angle as in the 

 homocercal tail. This symmetry in the cod, however, is not 

 primitive, but is due to the loss of the original heterocercal tail, 

 and the meeting of dorsal and ventral fins at the end of the 

 vertebral column. 



The endoskeletal supports of the paired fins likewise 

 undergo reduction in the bony fishes in increasing degree as 

 we pass from the Ganoids to the Teleosteans. The pectoral 

 girdle is reduced in the latter to two small bones called scapula 

 and coracoid, and these are attached to the inner side of a 

 series of dermal bones which perform the function of a pectoral 

 arch. The most primitive Ganoids, called for this reason 

 Crossopterygians, have the endoskeletal basals and radials well 

 developed, as in sharks, and supporting a fleshy lobe fringed with 

 the dermal rays {crossos means fringe). In other bony fishes 

 the basals and radials are reduced and the dermal rays 

 appear to rise from the base of the fin. There are no 

 dermal bones in the pelvic girdle, the original girdle is 

 much reduced, and the original basals form a single bone. 

 One of the most curious modifications in Teleosteans is the 

 change in position of the pelvic fins, which have moved 

 forward so that they are in the most specialised forms either below 

 or actually in front of the pectorals ; their skeleton is in these 

 cases actually attached to the pectoral girdle, for example 

 in the perch or cod. In such fishes, therefore, we have 

 vertebrate animals whose hind limbs are on the same level or 

 actually in front of their fore limbs. A cod, for example, 

 may be truly said to have its hind legs attached to its throat. 



The gill-clefts in the dog-fish and its allies open separately 

 on the surface of the skin and are separated from each other by 

 broad partitions or septa, at the inner borders of which are the 

 cartilaginous gill-arches. The internal openings of the gill- 

 clefts lead into the cavity of the throat, or as it is technically 



