MAMMARY GLANDS IN MONOTKEMATA. 763 



scent-glancls at the sides of the abdomen in Shrews, which are most 

 active at the season of the rut. 1 I put this question to the test, 

 first by showing the true structure of the mammary lobules, and 

 next by comparing the relative size of the glands with the con- 

 dition of the ovaria. 2 The abdominal scent-glands are present in 

 both sexes, and become largest in the male Shrews : but, in the 

 Ornithorhynchus the glands are confined to the female, and vary 

 in degree of development at different periods in individuals of equal 

 size, attaining an enormous development after gestation and being 

 small at the rutting season. The secretion being conveyed out- 

 wardly by means of numerous long and narrow ducts indicates its 

 fluid nature, and is contrary to the mode in which odorous sub- 

 stances are excreted. The excretory orifices are by no means 

 extended over so wide a space, in proportion, as in the Shrew, 

 but are collected into one which accords with the size of the 

 mouth of the young animal, and this spot is situated in a part of 

 the body convenient for the transmission of a lacteal secretion 

 from the mother to her offspring. 



Compared with an ordinary mammary gland, that of the 

 Ornithorhynchus differs chiefly in the absence of the nipple, and, 

 consequently, of the surrounding vascular structure necessary for 

 its erection. But the remarkable modification of the mouth in 

 the young Ornithorhynchus removes much of the difficulty which 

 previously attached itself to the idea of the possibility of an 

 animal with a beak obtaining its nutriment by suction. The 

 width of the mouth in the smallest observed Ornithorhynchus, 

 fig. 600, corresponds with the size of the mammary areola ; and 

 the broad tongue, extending to the apices of the broad, short, and 

 soft jaws, fig. 601, with the fold of integument continued across 

 the angle of the mouth, are all modifications which prepare us to 

 admit such a co-aclaptation of the mouth of the young to the 

 mammary outlet of the parent as, with the combined actions 

 of suction in the recipient, and compression of gland in the 

 expellent, to effect this essentially Mammalian mode of nourish- 

 ment. 



The circumstances which first attract attention in these singular 

 objects, fig. 600, are the absence of hair, the soft flexible condition 

 of the mandibles, and the shortness of these parts in proportion 

 to their breadth as compared with those of the adult. The in- 

 tegument with which the mandibles are covered is thinner than 

 that which covers the rest of the body, and smoother, presenting 



1 xovi''. p. 457. 2 xxxvi'. 



