Mammals to Aquatic Life. 159 



observe in the integument of the former are therefore compa- 

 rativeij small. Thej retain a thick fur of short close-ljing 

 hair, which, being well impregnated with grease, preserves 

 the skin from saturation and also from loss of moisture when 

 the animal leaves the water. In consequence of the more 

 equable temperature of the water the shedding of the coat does 

 not proceed so vigorously as in those of the nearest allies of 

 these creatures on land. 



Now while in the case of those animals which only seek the 

 water for short periods the hair is quite efficient for the 

 purpose of regulating the radiation of heat from the body, it 

 is not so in the case of those which remain in the water for a 

 longer time. Water is a better conductor of heat than air, 

 and we therefore see how animals, especially where the tem- 

 perature of the water is low, as in the polar regions, are 

 clothed with a layer of fat lying beneath the cutis, which 

 checks the radiation of heat better than could a coat of hair. 

 The more absolutely the animal adapts itself to the life in 

 water the less will be the use of the hairy covering, owing to 

 the shortness of the stay on land ; and, as a matter of fact, 

 in the order of seals, hand in hand with the biological obser- 

 vations of the longer or shorter time spent on land by the 

 various species, we can determine the presence of a denser 

 covering of hair or detect a thinning of the coat, corresponding 

 with the gradual increase of the layer of blubber. 



In those mammals which live entirely in tiie water the coat 

 of hair disappears. Thus adult Sirenians exhibit only scanty 

 vestiges of such a covering, while their embryos still exhibit 

 traces of a thick coat of hair extending over the whole body, 

 including the limbs and tail. 



In two embryos of Manatus, respectively 11 and 26 centim. 

 long, I find tiie following appearance : — The integument of 

 the smaller of the two is still smooth, with tiie exception of a 

 number of regularly arranged small brown papillae, which are 

 gathered together on both sides of the upper lip ; in the lai'ger 

 specimen fine stiff hairs project from tiie papillte, wliich are 

 disposed in longitudinal rows, and the external coat now 

 acquires a resemblance to that of the Indian elephant, in 

 which likewise stiff hairs about 1 centim. in length are 

 arranged at certain intervals from one another in longitudinal 

 rows; but besides this, the skin of the embryo of Manatus 

 exhibits a vast number of very fine openings between the 

 papilla3, which must be regarded as the mouths of follicles 

 belonging to finer hairs. The anterior extremities and tlio 

 tail also exhibit this hairy covering. On both sides of the 

 upper lip the vibrissas, which persist in the adult, have grown 



