554 ORIGIN OF MAMMALS xix. 9 



the tarsus of the male, served by a gland in the thigh. It is possible 

 that this is used to immobilize the female during coition. 



The brain is relatively large and arranged essentially on the mam- 

 malian plan (Fig. 329). The pallial portion of the cerebral hemispheres 

 is well developed, not the striatal portion as in reptiles, and the surface 

 is actually convoluted in spiny ant-eaters, though smooth in the 



J 



fibr 



Fig. 328. Comparison of the cloaca and penis of tortoise (a) with monotreme 

 (b and c). The penis is shown erect in B, withdrawn in c. (From Ihle.) 



bl. bladder; cl. cloaca \fibr. corpus fibrosum; p.s. preputial sac; spong. corpus spongiosum; 

 sp.d. sperm duct; urogen. urogenital canal; a.c. urinary canal; v.d. vas deferens; w. ureter. 



platypus. Perhaps, therefore, the large brain and active, memorizing 

 habits appeared early in the Mesozoic, but an interesting feature is that 

 there is no corpus callosum joining the hemispheres. 



The soft parts also show mammalian characteristics. The diaphragm 

 is fully developed and the heart and single left aortic arch resemble 

 those of other mammals. These animals have therefore advanced in 

 their circulatory system beyond the anapsid condition, such as is 

 probably shown today in Chelonia (p. 397). 



With all their archaic features the monotremes also show many 

 specializations. The platypus is highly modified for aquatic life. Apart 

 from its bill there are the webbed feet, dorsal nostrils, long palate, 



