AQUATIC MAMMALS 



sively, apparently alternately, while the hind feet are extended laterally 

 and used, with the tail, as equilibrating apparatus. 



We cannot rely too greatly upon the internal anatomy of the platy- 

 pus in judging its changes for a life in the water; and we cannot even 

 be sure that the flatness of the body was not inherited from terrestrial 

 ancestors. The form of its feet, tail, "bill" and external ear are the re- 

 sult of this life, indubitably, but we cannot be sure of its muscles, bones, 

 and certain details of its internal organs. It and the echidna are the 

 sole representatives of an exceedingly primitive group of egg-laying 

 mammals, and we have no suitable material, either living or fossil, with 

 which to compare it. As these are the most primitive of mammals the 

 stock must be of tremendous age, and it is extremely likely that the 

 platypus has been aquatic for a great many millions of years. It would 

 doubtless be far more specialized in this direction but for the fact that 

 it is most at home along streams of but moderate size, has few if any 

 aquatic competitiors, and feeds chiefly upon food that is relatively in- 

 active. It is not impossible that its aquatic preferences began even be- 

 fore the placental mammals became differentiated. For the above rea- 

 sons the internal anatomy of the platypus will either be entirely omitted 

 or discussed with great circumspection. 



MARSUPIALIA 



Chironectes water-opossum or yapok: of Central America and 

 northern South America. It subsists chiefly upon small fish and crus- 

 taceans, but is attracted by almost anything edible. Its pelage is short 

 and dense and the external ears are well developed. The tail is round 

 but the hind feet are large and very broadly webbed, in some toes to 

 the tip and in others to the last phalanx. 



But few details regarding the habits of the yapock the only aquatic 

 representative of the Marsupialia are known, but its dependence upon 

 the land appears to be great. The development of the hind feet seems 

 to be its only change for swimming, if we except the character of the 

 pelage, and it is remarkable that the feet should be so highly specialized 

 while the tail is perfectly terete. The base of the latter tapers gradually 

 from the body, as in many other marsupials. Although it is known that 

 small marsupial pouch-young can suspend breathing for many minutes 

 without harm and Carl Hartman has told me of a swimming opossum 

 (Didelphis) with live pouch-young having been captured, it would seem 

 that in Chironectes the female must curtail aquatic activity while she 

 raises a family. Without doubt this factor has been of exceeding im- 



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