PISCES 



159 



genous sheath, and as they expand these processes meet and 

 fuse to form centra or bodies around the notochord and arches 

 around the neural canal. In the skate the perichordal sheath 

 of the notochord is chondrified early, and the rings of cartilage 

 thus formed are the centra. The neural and haemal processes 

 are formed independently, and secondarily fused with the 

 centra and with one another. Even in the case of the species 

 in which arch centra are developed the inner sheath of the 

 notochord is invaded by the cartilage. 



The rings around the notochord become thickened by 

 cartilaginous development, and the notochord is correspondingly 

 reduced. The constriction of the notochord takes place, how- 

 ever, only in the middle of each ring, and the cartilage is further 



Neural spine 



Neural plate 



Rib 

 Trans, proc. 



Neur. spine 

 Neur. proc. 



Haem. proc. 

 Haem. spine 



Centrum Neural process 



FIG. 78. Raia. The vertebral column of R. radiata at the hinder end of the 

 body and the beginning of the tail, with end views of a body and a tail 

 vertebra. 



strengthened by calcification. The centra thus take the shape 

 of ring-shaped bodies, hollow at each end, and such centra 

 are described as amphicoelous. The gelatinous notochord is 

 thus expanded between the centra and contracted to a thread 

 in each centrum. 



The neural canal is invested by the neural processes and 

 by other paired and median plates. The neural arch processes 

 are perforated by the ventral roots of the spinal nerves. Later- 

 ally a series of plates are formed, neural plates, and these are 

 each perforated by the dorsal root of the spinal nerve. The 

 arch surrounding the canal is completed by roofing spines. 

 The neural spines and the neural processes are in some of the 

 rays, at least, connected by bars which alternate with the neural 

 plates. The haemal processes developed in association with 

 the neural processes form transverse processes in the trunk 

 region which usually bear small ribs. Behind the body cavity 

 the haemal arches project downwards, and an arch, the haemal 



