VEETEBE.E OF VERTEBEATA. 



431 



trunk, the lower arches are divided into ribs, and supporting 

 transverse processes (parapophyses). In the tail of the Selachii and 

 Ganoidei they are continuously connected with the centrum, and run 

 out into spinous processes, just like the upper arches. 



In the Teleostei the costiferous transverse processes gradually 

 converge, in the caudal region, and form inferior arches, which are 

 not homologous with those of the Selachii and Ganoidei, although 

 they also form spinous processes. 



In the Chimser^, Dipnoi, and many Teleostei, the caudal portion 

 of the vertebral column ends by gradually diminishing in size, but in 

 most fishes it presents great modifications, which are correlated with 

 the development of the caudal fin. These modifications first affect 

 the lower arches, which, in the Sharks, form spinous processes, which 

 are greatly widened out at their ends, and with which the caudal fin, 

 which is most developed in its ventral region, is connected. In many 

 Sharks, and still more in the 

 Sturiones, this caudal skeleton 

 is differentiated in an unequal 

 fashion. The inferior spinous 

 processes are more largely de- 

 veloped; this is correlated with 

 the degeneration of the superior 

 spinous processes, and of the 

 superior arches of the terminal 

 caudal vertebrte; this of course 

 produces an up -turning of the 

 caudal end of the vertebral 

 column; and in this way the 

 inferior lobes of the caudal fin 

 of the shark get to be terminal 

 in jjosition. 



In the Teleostei this up-tui'n- 

 ing affects also the axial portion of the vertebral column. As a 

 number of the terminal centra of this column are generally fused 

 together, and, like their upper arches, feebly or not at all developed, 

 while their inferior arches still persist, the up-turning must be the 

 more marked in proportion as the inferior arches become more 

 numerous and larger than the superior ones. This condition 

 (Fig. 224) is carried still farther by the atrophy of a large number 

 of vertebra, so that nothing remains of them but their inferior 

 arches (Physostomi) . 



Finally, the vertebra3 completely disappear, and the remams of 

 the inferior arches of the caudal region are connected, m the form of 

 vertical plates, with a single vertebra which represents the end of 

 the vertebral column; a style-shaped process (urostyle) of the 

 column is directed upwards, and contains the end of the notochord 

 (Acanthopteri). 



Supporting organs, formed from the integument, are connected 

 with the parts thus formed by the vertebral column, and they 



Fig. 224'. End of the caudal portion of 

 the vertebral column of a young Cypri- 

 noid. V Centrum. 71 Superior; h In- 

 ferior arches (the cartilaginous parts 

 are dotted), c End of the notochord. 

 d Covering bony lamella, r Bony rays 

 of the caudal fin. 



