ORGANS OF SUPPORT, 



two rib-like iliac bones, and placed on the lower part of the 

 trunk, sometimes near the anus, sometimes near the head, 

 or between these two parts. The iliac bones not being 

 here attached to the vertebral column, there is no portion 

 of that column fixed to form a sacrum, and the same is 

 observed in the cetacea, and the perennibranchiate am- 

 phibia, where there is no sacrum, and the whole column 

 behind the head is thus free for the extensive motion re- 

 quired in swimming. These two pelvic bones (80,) are 

 sometimes closely applied to each other, extend along 

 the middle of the abdominal surface, like two pubic bones, 

 and are attached to the scapular arch, or to the humeri (48,) 

 as in the perch. In the apodal fishes, as the eels, the pelvic 

 bones (80,) are wanting, as well as the legs ; and in the 

 abdominal fishes, the pelvic bones are quite unconnected 

 with the skeleton. From this freedom of the posterior 

 members in fishes, they are most frequently placed forward, 

 near the head, where they afford least impediment to the 

 lateral motions of the vertebral column. The long pha- 

 langes of the feet (81, 82,) are attached directly to the 

 pelvic bones, there being seldom a trace of the intermediate 

 bones of the legs developed in this class, where they are 

 not required either to give support to the trunk, or mobility 

 to the feet. 



In the plagiostome cartilaginous fishes, the highest ani- 

 mals of this class, the ribs and the spinous processes of 

 the vertebrae of the trunk, are as little developed as in am- 

 phibia, many of the anterior vertebrae (e 9 ) of the trunk, 

 are often anchylosed, and the whole bones of the cranium 

 are united into a single 

 piece, as seen in the 

 skeleton of the common 

 skate, raia betis, ( Fig. 

 32.) The skull (,) is of 

 great size, with tough, 

 thick, cartilaginous pa- 

 rietes ; it is filled chiefly 

 with the soft, glary, 

 arachnoid tissue, and 

 contains within the thick- 

 ness of the temporal bone 



FIG. 32. 



