THE PLACENTA 125 



a special sucking mouth. This sucking mouth is an extra-uterine 

 production, and is of course an adaptation to the particular needs 

 of the young, just as are other larval organs, such as the chin- 

 suckers of the tadpole, or the regular ciliated bands of the 

 larvae of various marine invertebrate organisms. 



There are a number of other features which distinguish the 

 Marsupials from other mammals. 



The cloaca of the Marsupials is somewhat reduced, but is still 

 recognisable. Its margins in Tarsipes are even raised into a 

 wall, which projects from the body. 



The tooth series of the Marsupials was once held to consist of 

 one dentition only, with the exception of the last premolar, which 

 has a forerunner. The interpretation of the teeth of Marsupials 

 are various. Perhaps most authorities regard the teeth as being 

 of the milk dentition, with the exception of course of the single 

 tooth that has an obvious forerunner. But there are some who 

 hold that the teeth are of the permanent dentition. In any case 

 it is proved that a set of rudimentary teeth are developed before 

 those which persist. Those who believe in the persisting milk 

 dentition describe these as prelacteal. Another matter of im- 

 portance about the teeth of this order of mammals is that their 

 numbers are sometimes in excess of the typical Eutherian 44. 

 This, however, holds good of the Polyprotodonts only. 



It was for a long time held that the Marsupials differed 

 from all other mammals in having no allantoic placenta. But 

 quite recently this supposed difference has been proved to be 

 not universal by the discovery in Perameles of a true allantoic 

 placenta. The Marsupials have been sometimes called the Di- 

 delphia. This is on account of the fact that the uterus and the 

 vagina are double. Very frequently the two uteri fuse above, 

 and from the point of junction an unpaired descending passage 

 is formed (see Fig. 48 on p. 74). 



A character of the brain of Marsupials has been the subject 

 of some controversy. Sir Kichard Owen stated many years ago 

 that they were to be distinguished from the higher mammals by 

 the absence of the corpus callosum. Later still it was urged 

 that a true corpus callosum, though a small one, was present ; 

 while, finally, Professor Symington l seems to have shown that 



1 "The Cerebral Commissures in the Marsupialia aiid Monotremata," Journ. 

 Anat. Phys. xxvii. 1893, p. 69. 



