THE LOWER VERTEBRATA. 



179 



cardinal it represents the rabbit's azygos vein. A caudal 

 vein, bringing blood back from the tail, splits behind the 

 kidneys, and forms the paired renal portal veins, breaking 

 up into a capillary system in the renal organ. A portal 

 vein brings blood from the alimentary canal to the liver. 



The blood of the dogfish contains red and white corpuscles, 

 both of which are almost identical, in size and form, with 

 those of the frog. 



Fig. 93. SKULL, VISCERAL ARCHES, AND PART OF VERTEBRAL COLUMN 

 OF DOGFISH. 



(Modified from Marshall and Hurst.) 



11. The Skeleton (excluding the placoid scales) is^ 

 almost entirely cartilaginous, though some of the parts are 

 calcified. Hence only those parts which are cartilage or 

 cartilage-bone in frog and rabbit are represented here. 



The vertebral column (fig. 93), consists of a very large 

 number of vertebrae, divisible into two sets only trunk- 

 and tail-vertebrae. No sacral vertebrae can be recognized, 

 as the pelvic girdle does not meet the vertebral column. 

 In a typical trunk vertebra we have a centrum, roughly 

 cylindrical in shape, and with a deep conical hollow at 

 each end the two hollows meeting in the middle. Thus a 



