XIII 



PHYLUM CnoKDATA 



75 



P^2/ 



I pc 



the series of Craniata we find every ^i-udation fVoin the persistent 

 notochord of the Cyclostoniata, through the imperfectly differen- 

 tiated vertebrae of Sharks and Kays, to the complete bony 

 vertebral column df the higher forms. 



The vei-tebra- are equal in number to the myomeres, but are 

 ai-ranged alternately with tht'm, the fibrous partition between two 

 myomeres abutting aoainst the middle of a vertebrae so that each 

 muscle-segment acts upon two adjacent vertebra?. Thus, the 

 myomeres being metameric 

 or segmental structures, the 

 vertebrie are intersegmental. 



In connection with the 

 anterior end of the noto- 

 chord, where no vertebra^ are 

 foi'med, there are developed 

 certain elements of the hlcull 

 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 

 (ncli) and thus continuing 

 forward the line of vertebral 

 centra. In front of the para- 

 chordals are developed a pair 

 of curved cartilaginous rods, 

 the trabeculce (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 dyfy) 

 already referred to. Cartilaginous investments are also formed 

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

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

 organs of sight, and one of auditory capsules {cm. 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 trabeculte, and are continuous 

 with those structures fi'om an early stage. The auditory capsules 



1' 

 cczc.c 



7tch> 



Fii:. 703. — T}ie elements of the cTuniiim in an 

 embryo Salmon, from above, av. c.anditory 



capsule ; i\rh. notocliord ; [ir. parachordal : yty. 

 position of pituitary bodj' ; tr. trabcculii. (From 

 a model by Ziegler.) 



