ZOOLOGY 



SECT. 



of the anterior ribs in the middle ventral line. In this way the 

 anterior or thoracic region of the ccelome is enclosed in an articulated 

 bony framework formed of the vertebral column above, the ribs at 

 the sides, and the sternum below. The ribs under these circum- 

 stances become segmented each into two parts, a dorsal vertebral 

 rib, articulating with a vertebra, and a ventral sternal rib with the 

 sternum. In the tail there is frequently a liwmal arch (Fig. 715, D, 

 h. a.) springing from the ventral aspect of the centrum and en- 

 closing the haemal canal. Thus the line of centra in the fully 

 formed vertebral column occupies the precise position of the 

 notochord ; the neural arches encircle the spinal portion of the 

 cerebro-spinal cavity ; the transverse processes, ribs, and sternum 

 encircle the coelome ; and the haemal arches similarly surround 

 the haemal canal or vestigial coelome of the tail. As we ascend 

 the series of Craniata we find every gradation from the persistent 

 notochord of the Cyclostomata, through the imperfectly differen- 

 tiated vertebrae 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 parti- 

 tion between two myomeres 

 abutting against the middle 

 of a vertebra, so that each 

 muscle-segment acts upon 

 two adjacent vertebrae. Thus, 

 the myomeres being meta- 

 meric or segmental struc- 

 tures, the vertebrae are inter- 



seginental. 



In connection with the 

 anterior end of the noto- 

 chord, where no vertebrae are 

 formed, there are developed 

 certain elements of the skull 

 or cephalic skeleton, a struc- 

 ture eminently characteristic 

 of the whole craniate divi- 

 sion, and to the possession of 

 which, indeed, it owes its 

 name. The skull makes 

 its first appearance in the 



embryo in the form of paired cartilaginous plates, the parachordals 

 (Fig. 718, pc), lying one on each side of the anterior end of the 

 notochord (nek) and thus continuing forward the line of vertebral 



7i ch, 



FIG. i IS. The elements of the cranium in an 

 embryo Salmon from above, an. c. auditory 

 capsule; nch. notochord; pc. parachordal ; pti/. 

 position of pituitary body ; tr. trabecula. (From 

 a model by Ziegler.) 



