PHYLUM CHOEDATA 



75 



the series of Cramata we find every gradation from the persistent 

 notochord of the Cyclostomata, through the imperfectly differen- 

 tiated vertebra of Sharks and Rays, to the complete bony 

 vertebral column of the higher forms. 



The vertebrae are equal in number to the myomeres, but are 

 arranged alternately with them, the fibrous partition between two 

 myomeres abutting against the middle of a vertebrte, so that each 

 muscle-segment acts upon two adjacent vertebrEe. Thus, the 

 myomeres being metameric 

 or segmental structures, the 

 xertebrse are intersegmental. 

 / In connection with the 

 anteriol^ end of the noto- 

 chord, where no vertebrae are 

 formed, there are developed 

 certain elements of the slcull 

 or cephalic skeleton, a struc- 

 ture which is eminently 

 characteristic of the whole 

 craniate division, and to the 

 possession of which it owes 

 its name. The skull makes 

 its first appearance in the 

 embryo in the form of paired 

 cartilaginous plates, the 

 parachordals (Fig. 763, pc), 

 lying one on each side of the 

 anterior end of the notochord 

 (nch) and thus continuing 

 forward the line of vertebral 

 centra. In front of the para- 

 chordals are developed a pair 

 of curved cartilaginous rods, 

 the trdbeculcB (tr), which un- 

 derlie the anterior part of 



the brain, as the parachordals underlie its posterior part : their 

 hinder ends diverge so as to embrace the pituitary body {pty) 

 already referred to. Cartilaginous investments are also formed 

 around the organs of the three higher senses : a pair of olfactory 

 capsules round the organs of smell, one of optic capsules round the 

 organs of sight, and one of audito7-y capsules (au. c.) round the 

 organs of hearing. The optic capsule, which may be either fibrous 

 or cartilaginous, remains free from the remaining elements of the 

 skull in accordance with the mobility of the eye ; it constitutes, in 

 fact, the sclerotic or outer coat of that organ. The olfactory capsules 

 are usually formed in relation to the trabeculse, and are continuous 

 with those structures from an early stage. The auditory capsules 



Cl,ZC.C 



nch. 



Fig. 763. — The elements of the cranium in an 

 embryo Salmon, from above, au. c.-auditory 

 capsule ; nch. notocliord ; pc. parachordal : piy. 

 position of pituitary body ; tv, trabccula, (B'rom 

 a model by Ziegler.) 



