466 CHORDATA. 
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 JZonotremata 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 (1) the upper part 
or Fallopian tube, (2) the middle part or wéerus 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 A/etatheria there is little or no fusion, so 
that there are two uteri and two vagine, but in the Eutheria 
the two vagine are always fused into one. Lastly, in all the 
higher Zutheria 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 Marsupdalia 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 perinzeum) and is more complex in other ways 
