466 CHORD AT A. 



are typically four chambers, of which the two first are non- 

 glandular, as is also the third (see Ruminantia, page 514). 

 It is difficult to find any general law regulating the amount 

 of complexity of the stomach. In a very wide sense, the 

 carnivorous animals have the simpler and the herbivorous 

 have the more complex stomach, but there are many excep- 

 tions to these, such as the whales. 



The intestine is usually long in the herbivorous mammals 

 and comparatively short in carnivorous, and the same applies 

 especially to the caecum which may be entirely absent in 

 certain Carnivora. 



Lastly, we may notice that in the great majority of 

 mammals the anus opens to the exterior independently of 

 the urogenital sinus, no cloaca being present. 



Urogenital System. — The urogenital organs show a 

 transition series as the viviparous habit is acquired and 

 elaborated. In the oviparous Monotremata the oviducts are 

 like those of reptiles, simple throughout and opening 

 separately into the urogenital sinus. In the higher types 

 the oviduct becomes differentiated into (i) the upper part 

 or Fallopian tube^ (2) the middle part or uterus and (3) the 

 lower part or vagina. At the same time there takes place 

 a fusion of the two oviducts in the middle line. In the 

 majority of the Metatheria there is little or no fusion, so 

 that there are two uteri and two vaginae, but in the Eutheria 

 the two vaginae are always fused into one. Lastly, in all the 

 higher Eutheria the two uteri are more or less fused into 

 one, transition forms giving rise to the types of uterus called 

 bicornuate and bi-bipartite. 



In the male there is a corresponding progress in the evolution of the 

 penis and the urogenital system generally. It is evident that the 

 viviparous habit requires a complete internal fertilisation, even more than 

 in the terrestrial oviparous forms. The penis in the Sauropsida is 

 merely the specialised ventral wall of the cloaca, which is only partially 

 protrusible ; on its dorsal surface is a groove, the penial urethra. In the 

 Monotremata the penial urethra has become a tube along the dorsal sur- 

 face of the penis, which, however, communicates freely behind with the 

 cloaca as well as with the urogenital sinus. In the Marsupialia the 

 urogenital sinus and the penial urethra are continuous and completely 

 apart from the rectum, but the distal end of the penis is still surrounded 

 by the same sphincter muscle as the anus {cf. female), whereas in the 

 Eutheria the penis is perfectly distinct and free from the anus (the space 

 between the two being the perin?eum) and is more complex in other ways 



