558 



CHORDATA 



others viviparous, but are distinguished by the duration of pregnancy. 

 The eggs of the viviparous species 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 

 Monodelphia, Placentalia. 



.Ml mammals care for the young, 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 polygamous, while in others there is no 

 permanent association of the sexes. The body temperature is constant and 

 ranges from 3 6_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. Monotremata (Ornithodelphia, Prototheria). 



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 Ornithorhyn- 

 chns in a nest, by Echidna in a temporary 

 pouch (marsupium) on the ventral sur- 

 face of the body. On hatching the 

 young are nourished by the secretion of 

 --& enormously enlarged sweat glands, which 

 form two large masses to the right and 

 left of the mid-ventral surface. Each 

 FIG. 606. Pelvis (left side) of opens on a special region of the ventral 



8S3J233" 1 ftSSSj ^^ Whkh is ^^ in Ornithorhyn- 

 amen; //, ilium; is, ischiunv Om c ^ lus , a flattened pocket in the others. 



>one; P, os pubis. Other distinctions from other mam- 



mals, which are also points of resem- 



>lance to reptiles and birds, are the strong development of the epi- 

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



