XIII 



PHYLUM CHORDATA 



147 



those of the higher forms, are developed as chondrifications of the 

 sheath of the notochord into which cells of the skeletogenous layer 

 have migrated (p. 73). On the dorsal side of the row of centra the 

 spinal column is represented by the series of neural arches which 

 support the walls of the spinal canal. Owing to the presence of a 

 series of intercalary cartilages the neural arches appear to be 

 twice as numerous as the central Each neural arch consists on 

 each side of a process, the neural process, given off from the 

 centrum, and of a small cartilage, the neural plate (basi-dorsal), 

 which becomes completely fused with the neural process in the 



n.c 



n.sp 



n.a 



h. 



Fia. 812. Portions of the vertebral column of Scy Ilium canicula. A and B, from the 

 trunk ; C and D, from the middle of the tail ; A and C, two vertebra in longitudinal section ; 

 B and I), single vertebrae viewed from one end. li, calcified portion of centrum ; c 1 . centrum ; 

 ./>>/. foramen for dorsal, and ]','. for ventral root of spinal nerve ; IL.OK haemal .arch (basi- 

 venfral) ; h.c. haemal canal; li.xj). haemal spine; i.n.p. intercalary piece (interdorsal, 

 or interneural plate); ii.a. neural arcn x ; /i.e. neural canal; ,i.t>. neural plate (basi-dorsal) ; 

 /'.*/<. neural spine; ntc. intervertebral substance (remains of notochord); r. proximal portion 

 of rib ; tr.pr. transverse process (basal stump).* (From Parker's Practiced Zooloyy.) 



adult. Between successive neural plates, the width of each of 

 which is only about half the length of the centrum, are interposed 

 a series of plates of very similar shape, the interdorsal or inter- 

 neural plates. Small median cartilages, the neural spines, fit in 

 between both neural and interneural plates of opposite sides and 

 form keystones completing the arches. 



The transverse processes are very short : connected with each of 

 them is a rudimentary cartilaginous rib (r.) about half-an-inch in 

 length. 



The cranium (Fig. 813) is a cartilaginous case, the wall of which 

 is continuous throughout, and not composed, like the skulls of 

 higher Vertebrates, of a number of distinct bony elements fitting 

 in together. At the anterior end is a rostrum , consisting in Scyllium 



K 2 



