55S 



CHORD ATA 



others viviparous, 1)Ut are distinguished Ijy the duration of pregnancy. 

 The eggs of the ^•iviparous s])ecies are so small (about .01 inch) that they 

 have a total, nearly equal segmentation. Such eggs require nourishment 

 from the mother in order to produce an animal with the complicated 

 structure of a mammal. Since in the Didelphia the uterine nourishment 

 is usually very incomplete, the period of pregnancy is very short, in com- 

 parison with the Monodelphia, in which a placenta, a complicated apparatus 

 for the nourishment of the young, appears; hence the marsupials, with 

 their small imperfectly formed young, are of en called Aplacentalia; the 

 JMonodelplua, Placentalia. 



All mammals care for the voung, this being chiefly or wholly done by the 

 mother, who not only supplies them with milk but protects them in nests. 

 Most mammals are monogamous, some j)olygamous, while in others there is no 

 permanent association of the se.xes. The body temperature is constant and 

 ranges from 36° to 41° C. (98° to 106° F.); in Echidna it is only 26° to 34° C. 

 (79° to 83° F.). In most, continual feeding is necessary for existence; from this 

 rule there are a few exceptions, like the bears, marmots, badgers, etc., which 

 hibernate during the winter, taking no food. .At this time there is a fall in 

 the temperature (in the marmots nearly to freezing) due to the diminished 

 metabolism. 



Sub Class I. Monoirentata {Oniilhodelphia, Prototlicria). 



A few mammals, confined to Australia and New Guinea, are the only 

 living representatives of the group. They are distinguished from all 



other mammals by laying eggs about 

 half an inch long, rich in yolk and with 

 soft shells. These undergo in the uterus 

 a discoidal (meroblastic) segmentation 

 and are then incubated by Omillwrli vn- 

 clnis in a nest, by Echidna in a temporary 

 pouch (marsupium) on the ventral sur- 

 face of the body. On hatcliing the 

 young are nourished by the secretion of 

 j^ enormously enlarged sweat glands, which 

 form two large masses to the right and 

 left of tire mid-ventral surface. Each 

 opens on a special region of the ventral 

 surface, which is slit-like in Ornithorliyn- 

 cliiis, a llattcned pocket in the others. 



Other distinctions from other mam- 

 mals, which are also points of resem- 

 blance to reptiles and l)irds, are the strong development of the epi- 

 sternum and the extension of the coracoid to the sternum (fig. 599), the 



Fig. 606. — Pelvis (left siilc) of 

 Ornilhorhynchus paradoxus (from 

 WiedersheUTi). Fn, ohturalnr for- 

 amen; II, ilium; /\, ischium; Om, 

 marsupial bone; P, os pubis. 



