PLACENTA. HABITS. 533 



In both the male and female the openings of the imnogenital sinus 

 and anus are enclosed by a common sphincter, so that there is a shallow 

 cloaca. In the male this sphincter is said, by compressing the veins 

 of the penis, to exercise an important influence on its erection. 



A marsupial pouch is present in most forms. It is however absent 

 in Didelphys and the Dasyuridae. Its opening generally looks forward, 

 but in Thylacinus and some Peramelidae it is directed backwards. The 

 number of teats bears a relation to the number of yomig produced at a 

 birth. Tlie kangaroo, with one young, has four nipples. The Virginian 

 opossiun with several yoiuig has six on each side and one in the middle. 

 In Phascoloqale penicillata there are eight nipples arranged in a circle. The 

 young are carried in the mouth to the pouch and are attached by their 

 mouths to the nipples. The nipples are long and the milk is forced down 

 the mouth of the very imperfect yoiuig by the contraction of the cre- 

 master muscle. The epiglottis and arytenoid cartilages are elongated 

 and project into the posterior nares in very much the same fashion as in 

 whales. In this way the young are able to breathe while milk is being 

 forced down their throats. In the pouched forms the young reti.u'n to 

 the pouch for nutrition and shelter after they have acquired the power 

 of locomotion. A rudiment of the pouch is occasionally present in the 

 male. 



The ripe ovum appears to be of about the same size as that of liigher 

 mammals (in Phascolarctus 0*1 7 mm.). The yolk sac becomes in part 

 vascular; its outer epithelium and that of the false amnion become 

 closely applied to the uterine wall. In some cases at least an epithelial 

 attachment * is formed, but as a rule no closer cormection is established. 

 The majority of the marsupials are therefore truly aplacental, but in 

 Perameles f a definite discoidal (primary) allantoic placenta is developed. 

 The young of Perameles at birth do not differ in degree of development 

 to any marked extent from other marsupials. 



In their external appearance, in their mode of nourishment, 

 and in their habits, marsupials differ much amongst themselves. 

 Some (e.g. kangaroos) are purely herbivorous and in their 

 dentition approach the rodents and ungulates ; others (thyla- 

 cines and dasyures) are carnivorous ; but most of them are 

 omnivorous. In their general appearance and mode of loco- 

 motion they repeat a series of types of different mammalian 

 orders. Thus the wombats represent the rodents : the fleet 

 kangaroos, which move by huge bounds, correspond to ruminants. 

 The flying marsupials {Petaurus) represent the flying squirrels 

 {Pteromys) ; the climbing phalangers (Phalangista), in their 

 shape and mode of life, recall the lemurs ; while others, as the 

 Peramelidae, show a likeness to the shrews (Soricidae) and 

 insectivores. Finally the carnivorous marsupials approach in 



* Caldwell, Q.J.M.S., 24, 1881. Selenka, Studien iih Entwich. d. 

 Thiere, iv, Da-s Opossum ; v, Phalangista u. Hypsiprymnus, Wiesbaden, 

 1886-91. 



t Hill, Q.J. M.S., 48, 1808, p. 385. 



