16 



(b) DiDELPHiA (Metatheria). — Characters. — Teats present. No placenta 



Marsupial bones. Young born imtnaturely. Coracoids rudimentary. 

 True teeth. Usually more upper than lower incisors. 



II. MAESUPIALIA (pouclied animals). —The only order of didelph 

 mammalia. Marsupials have the characters of Didelphia, plus the following : 

 — Marsupial pouch, into which the teats project ; and young are attached to 

 teats for considerable time after birth. Two uteri ; two vaginBe. Scrotum 

 in the male in front of the penis. Angle of the mandible is inflected (except 

 in one case). Eadius and fibula separate from ulna and tibia. All, except 

 the opossums (Didelphidse), are confined to Australasia. Australasia was cut 

 off from connection with the other continents by the sea before the higher 

 mammals appeared. In the struggle for existence, the higher mammals 

 afterwards exterminated the marsupials, except in South America (opossums). 

 In Australasia no higher mammals were developed; the marsupials there 

 survived, and, owing to freedom from external competition, were able to 

 undergo specialisation. They specialised and developed along lines curiously 

 resembling those of the higher mammals. Thus there are marsupial 

 rodents {e.g. Wombat, Phascolomys), marsupial insectivores {e.g. Perameles), 

 marsupial carnivores {e.g. Dasyurus, Tasmanian Devil). 



(c) MoNODBLPHiA (Eutheria). — Mammalia with teats. Placenta. "Eo 



Marsupial bones. Coracoids rudimentary. Young born highly 

 developed. 



[The Placenta is an organ for the nutrition and respiration of the fcetus 

 during its development in the uterus ; it enables the mother to nourish the 

 young before birth, hence the placenta is a structure partly maternal and partly 

 fcEtal. The embryo of Sauropsida and Mammalia is invested by two sac-like 

 membranes — (1) the amnion, (2) the allantois. In Sauropsida the allantois 

 forms the embryonic respiratory organ. In Mammals the allantois serves a 

 nutritive function, helping to form along with a portion of the amnion the 

 fcetal part of the placenta, in the following manner : — The amnion is a double 

 envelope. The inner envelope of the amnion is the true amnion ; the outer, 

 the false amnion. A part of the allantois fuses with the false amnion to give 

 rise to the chorion. The placenta consists of vascular villi, i.e. fine small 

 fibres (foetal portion), which grow out from the chorion into corresponding pits 

 in the walls of the uterus (maternal portion). If at birth the foetal villi 

 simply escape from the uterine pits, the placenta is called non-deciduate. If 

 the connection between the ftetal villi and maternal pits is so intimate as to 

 make escape impossible, the greater portion of the lining membrane of the 

 uterus comes away at birth, and the placenta is called dcciduate. If the foetal 

 villi are scattered indefinitely around the embryo, the placenta is diffuse; if 

 they are gathered into clumps, the placenta is cotyledonary ; if in the form of 

 a girdle round the embryo, zonary ; if confined to a definite area, discoidal. 

 Thus there are the following types of placenta : — 1. iNDBCiDtJATE, which 

 may be — (a) Diffitse, e.g. Cetacea, Sirenia, most Ungulates, and Lemurs; 

 (6) Cotyledonary, e.g. sheep, cow 2. Deciduate, which may be — (a) Zonary, 

 e.g. Carnivora, Proboscidea ; {b)Discoidal, e.g. Insectivora, Rodents, Cheiroptera, 

 and Primates, except Lemurs. 



The monodelphs are called higher mammals, and include the following 

 ten orders, of which the least differentiated are the Insectivores (hedgehog, 

 mole, etc.), to which are most closely allied the Cheiroptera (bats) and the 

 Eodents (rat, rabbit, etc.) 



