AQUATIC MAMMALS 



carbonized blood by the venous ones, Turner and Milne-Edwards of re- 

 tarding the flow, and Murie was of the opinion that they facilitate inter- 

 change of substances with the lymphatic system. Hyrtl (1854) ad- 

 vanced the hypothesis that the diffuse pattern of retia is associated with 

 animals doing heavy muscular work but of an agile character, as with 

 the burrowing armadillo, while the cluster pattern is characteristic of 

 mammals in which movement is slow and posture prolonged, such as 

 the sloth. Wislocki (1928) has discussed this question at considerable 

 length and the reader desirous of further information is referred to 

 his paper. With the attention that the subject of retia is now attracting 

 it is to be hoped that we may soon know more about their useful func- 

 tions. At present, however, there is no strong evidence indicative of 

 what these may be. There is no evidence whatever that a retial condi- 

 tion of the blood vessels is useful for the storage of reserve blood. In 

 itself it would not assist in retarding the flow, nor is it likely that there 

 is interchange of substances with the lymphatic system within the retial 

 bundles, where these occur. In fact it seems that very little can yet be 

 said except that possibly a difl^use type of retia might largely overcome 

 any interruption of the blood flow that muscular or other pressure is 

 capable of producing. 



MAMMAE 



Before terminating the present contribution brief consideration should 

 be accorded the subject of the mammary equipment of aquatic mammals. 

 The way in which these are used is usually of slight consequence to a 

 terrestrial mammal, for except in particular cases as in some ungulates, 

 the young may nurse from any position. It is of critical import, however, 

 that an aquatic mammal shall be able before it forsakes the land en- 

 tirely to contrive a method whereby the young may suckle with reasonable 

 comfort while in the water. If this cannot be accomplished the creature 

 must either retain its connection with the land, with consequent impli- 

 cation of reduced ability to develop the highest aquatic modifications, 

 or else become extinct. 



Probably in no mammal has the aquatic life caused any definite altera- 

 tion in the position of the nipples, a possible exception being in the 

 Sirenia. In Cetacea and Pinnipedia there is one pair of mammae situ- 

 ated inguinally. In Sirenia there is a pair of axillary ones, but they now 

 occur practically upon the posterior border of the flippers. The coypu 

 (Myocastor) has two pairs that are situated almost upon the back, one 

 pair being just behind the shoulders and the other near the haunches. 

 They are largely similar in the capybara but rather less elevated, as is 

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