528 



MAMMALIA. 



genital sinus wliicli posteriorly opens into the cloaca. Anteriorly it 

 receives the openings of the bladder, vasa deferentia and vu'eters. There 

 is a penis attached to the ventral wall of the cloaca and retractile into a 

 sheath (Fig. 275). It is traversed by a canal (penial lu-ethra) wliich 

 ODens in front into the virinogenital sinus, and behind by rnore than one 

 opening at the end of the penis. There is a pair of large Cowper's glands 

 opening into the canal of the penis. The corpus fibrosuni is not cavernous, 

 but there is cavernous tissue round the urethra and in the glans penis. 

 The prostate and vesiculae seminales are absent. The ovaries are in the 

 same position as the testes, but the right ovary is smaller (never functional 

 in Ornithorhynchus, rarely in Echidna) than the left (Fig, 270). There 

 is no vagina. The lower ends of the ovidvicts are dilated into a kind of 

 uterus, and open into the urinogenital sinus in front of the vu-eter. There 



are two glands opening 

 into the sheath of the 

 clitoris which are com- 

 pared by Owen to the 

 Cowper's glands of the 

 male. 



The ova when they enter 

 the oviduct are larger than 

 in all other mammals (3 to 

 4 mm. ), and contain a con- 

 siderable quantity of yolk. 

 They acquire an albumi- 

 nous coat and a shell in the 

 oviduct, where they im- 

 dergo their incomplete 

 cleavage and the early 

 stages of de^'elopment. 

 The ovum increases con- 

 siderably in size in the 

 oviduct by the intussus- 

 ception of fluid and is 

 about 15 X 12 mm. when 

 extruded. 



In Echidna one egg is 

 laid at a tinae and placed 

 by the mother in the pouch (p. 526). Tn Ornithorhynchus two eggs 

 appear to be laid and placed in the nest in the burrow. 



The temperature * is lower (about 28-29° C.) than is usual in mammals 

 and appears to exhibit considerable variation. Echidna hibernates in the 

 cold weather. Fossil remains, closely allied to living forms, have been 

 found in the Pleistocene of Australia, and Ameghino t has ascribed some 

 fossils in the Eocene of Patagonia to this group [Dideilotherium, etc). 



Fam. 1. Echidnidae. Skin covered with spines with which hairs 

 are mingled ; snout elongated ; edentulous ; tongue long and protractile ; 

 salivary glands large ; tail very short. They burrow in sand and earth. 

 Echidna Cuv. (1798), with 5 clawed digits on each limb, with a temporary 



* Martin, Thermal adjustment and respiratory exchange in Mono- 

 tremes and Marsupials, Phil. Trans., 195, 1903, p. 1. 

 t Bol. Acad. Cordoba, xiii, 1894. 



Fig. 275. — Diagram of the cloaca, urinogenital simis 

 and penis of a Monotreme, A with the penis pro- 

 truded, B retracted, br connective tissue ; bl bladder ; 

 cl cloaca ; / corpus fibrosum ; p ureter ; ps sheath 

 (preputial) of penis; jfs' opening of this; r canal 

 traversing the penis (sperm tube) ; s vas deferens ; 

 u urinogenital sinus (after Boas). 



