xii.] THE EXCRETORY ORGANS. 489 



spicuous mark running antero-posteriorly along each side of 

 the body and tail of most Fishes. 



That special modification of the cutaneous glands called 

 the MAMMARY GLAND, or breast, is peculiar to man's class, 

 which from this circumstance alone bears the name of 

 Mammalia. 



These glands may be devoid of a nipple, as is the case in 

 the Ornithorhynchus and Echidna, or the nipples may be 

 very long, as in Marsupials. There may be as many as 

 twenty-two glands, as sometimes in 

 Centetesj or ten, as in the Hedgehog 

 and domestic Sow. 



There is never one nipple only, 

 though very rarely their number 

 may be odd, as in some Opossums. 



The glands may open into a little 

 depression, or sac, formed during 

 their functional activity, as in the 

 Echidna. FlG 4I0 ._ MARSUP i AL POUCH 



I hey may be placed inside a OF AN OPOSSUM (Didetykys 

 permanent cutaneous pouch, as in cancrivor a \ cut open to the 



\, . , eleven nipples within one of 



Marsupials. these eleven teats being me 



In that group there may be four dian in position, 

 nipples, as in the Kangaroo ; or 



eight arranged in a circle, as in Phascopale penicillata; or 

 four on each side and one in the middle, as in Didelphys 

 dorsigera ; or six on each side and one in the middle, as in 

 the Virginian Opossum. 



These glands may be placed nearer the middle line of the 

 back than that of the belly, as in the aquatic Coypu Rat ; 

 or near the armpit, as in Pteropus. 



There may be two pectoral mammae in animals remote 

 from man's order, as in the Elephant, Sloths, and Sirenia. 



The Sirenia from this circumstance, together with their 

 rounded heads and fish-like tail, probably gave rise to the 

 belief in the existence of Mermaids. 



There may be but two mammary glands, each opening in 

 a depression and placed quite at the hinder end of the belly, 

 as in Cetaceans. 



In Ruminants we have posteriorly situate glands with 

 teats forming an " udder." 



In man's own order we find, in the Ape, but two pectoral 

 mammae ; but amongst the Lemuroids there may be only a 

 pair of ventral (inguinal) glands, as in Chciromys j or a pair 



