\ni 



PHYLUM CfTOPvDATA 



147 



those of the higher forms, are dexeloped as choiKh-ificalioiis of the 

 sheath of the notoehord into which cr'lls of the .skeletogenous hiyer 

 have migrated Cp. 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 ot a 

 series of intercalary cartilages the neural arches appear to be 

 twice as numerous as the centra. Each neural ai'ch consists on 

 each side of a process, the neiiral pi'ocess, given off from the 

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

 which becomes completely fused with the neural process in the 



/•^/' A^ y. n.p J'P n. sp 



n.a 



n.c 



n.a 



h.a: 



Fio. 812.— Portions of the vertebral column of Scyllium canicula. A and /?, from the 

 trunk ; Cand D, from the middle of the tail ; A and (\ two vcrtcbi-iM in long-itudinal section ; 

 B and /), single vertebras viewed from one end. Ii, calcilicd portion of centrum ; <•. centrum ; 

 j'oi-. foramen for dorsal, and fn-'. for ventral root of spinal nerve ; li.a. hasmal-arch (basi- 

 ventral) ; li.t-. hajmil canal; /(..<</). hsemal spine; i.n.p. intercalary piece (interdorsal, 

 or interneui-al plate); ii.rt. neural arcn ; n.c. neural canal; n.p. neiiral plate (basi-dorsal) ; 

 H..XI). neural spine ; ntf. intervertebral substance (remains of notoehord); v. proximal portion 

 of rib ; ti-.in-. transverse process (basal stiunp). (From Parker's Practical Zoolo'j//.) 



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 cranittm (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 



