THE MONOTREMES 



(MONOTREMATA). 



Non-placental mammals without true teeth, with the genital and urinary ducts opening along with the rectum into 

 a common chamber (cloaca). They have neither marsupial pouch nor teats, but have milk-glands and marsupial 

 bones. 



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This order, consisting only of two Austra- 

 lian genera, the duck-mole and the echidnas, 

 represents beyond doubt the lowest stage of 

 degradation that can be reached by the mam- 

 malian type. The structure of these remark- 

 able animals exhibits its many points of 

 resemblance to the marsupials, but there are 

 other characters peculiar to them which seem 

 to have been derived from reptiles, amphibians, 

 or fishes. 



The smooth brain of these animals is even 

 less developed than that of the marsupials, 

 and has, like that of the latter, an imperfect 

 corpus callosum and simple corpora quadri- 

 gemina with scarcely observable furrows. 

 The eyes are small, and the external ears 

 altogether wanting. The slit-like external 

 openings of the ears can be opened and 

 closed by the animal at pleasure. The bones 

 of the skull become fused, as in the birds, very 

 early. The teeth are either altogether want- 

 ing, or are represented, as in the duck-mole, 

 only by horny plates. The lower jaw is very 

 weak and has no inflexed angle. Through the 

 development of coracoid bones the shoulder- 

 girdle of the monotremes comes to resemble 

 that of the above-mentioned lower vertebrates. 

 The mouth has the form of a beak, in the 

 duck-mole flat and blunt, in the echidnas 

 rather long and pointed, and is not, as in 



almost all other mammals, clothed with fleshy 

 lips, but with a horny covering. The struc- 

 ture of the sexual organs is quite peculiar. 

 The ovaries produce large eggs, but that of 

 the right side is, as in the birds, degraded. 

 There is no true uterus. The egg remains 

 a long time in an enlargement of the oviducts, 

 which open separately along with the urinary 

 ducts into a common canal, the urino-genital 

 canal, which in its lower part receives also 

 the rectum, and thus forms a cloaca with a 

 single opening to the exterior. Such a struc- 

 ture, which exists also in the amphibians, 

 reptiles, and birds, is found also in the em- 

 bryos of the other mammals, in which, how- 

 ever, it disappears in the course of develop- 

 ment. 



The monotremes have five toes on all 

 four limbs. On the ankle of the hind-foot 

 (the tarsus) there is a sharp, horny spur sup- 

 ported by a few tiny bones, and this spur 

 might be regarded as a sixth toe. In the 

 females this spur is only rudimentary. In 

 the males it is traversed by a canal leading 

 into a gland lying beneath the skin of the 

 foot. The true function of this spur, which 

 has been erroneously regarded as a venomous 

 weapon, is not yet known. 



Quite recently it has been definitely shown, 

 especially by Haast and Caldwell, that the 



