306 



COMPARATIVE ZOOLOGY. 



by a more highly developed 

 vertebral column of cartilage 

 or bone. The column and 

 cranium are never absent in 

 the craniota ; other parts may 

 be VFanting, as the ribs in Frogs, 

 limbs in Snakes, etc.'" The 

 limbs are never more than 

 four, and are always articu- 

 lated to the haemal side of the 

 body, while the legs of Inver- 

 tebrates are developed from 

 the neural side. The muscles 

 moving the limbs are attached 

 to the endoskeleton. 



The circulation of the blood 

 is complete, the arteries being 

 joined to the veins by capil- 

 laries, so that the blood never 

 escapes into the visceral cav- 

 ity as in the Invertebrates. 

 All have a portal vein, carry- 

 ing blood through the liver; 

 all have lacteals and lym- 

 phatics. The blood is red, 

 and contains both kinds of 

 corpuscles.'" The teeth are 

 developed from the dermis, 

 never from the cuticle, as in 

 MoUusks and Articulates ; the 

 jaws move vertically, and are 



Fio. OT.-Diag,nn> oi Circulation m "^Ver modified limbs. The 



the higher Vertebrates : 1, heart; 2, liver and kiduevs are alwavs 



lungs ; 8, head and upper extremities ; •' . ■'. 



4, spleen; B, intestine; 6, kidney; 7, present. The respiratory or- 



lower extremities; 8, liver. (From •., .,, , 



Daiton'8 "Physiology.") gais ai'G either gills or lungs, 



