MAMMALIA. 549 



function ; it is probably to this subsidiary function that 

 they owe their preservation. 



The dorsal fin and the tail, except for its central vertebral 

 axis, have no osseous support like that of the paired fin, but 

 are stiffened by strong dense fibrous tissue. 



The heart in Cetacea is large, and there are underlying 

 the vertebral column a number of fine vessels, or retia 

 mirabilia, which may assist the animal in keeping under 

 water for long periods (see page 464). . 



It is sometimes asked, How do we know the porpoise 

 (^Cetacea) to be a mammal? And again. How is a porpoise 

 adapted for an aquatic habit? If we divide the structural 

 facts of the porpoise into (i) resemblances to other mam- 

 mals and into (2) adaptive characters, the questions will be 

 answered. Of the first category we have only to refer to 

 Table on page 43 1 and it will be found that the porpoise 

 agrees with all the twelve mammalian characters there 

 enumerated with the reservations of no hair, no hind- 

 limbs and homodont teeth. Again, it conforms to no 

 one character of the second class (fishes). 



Of adaptations to an aquatic habitat we may specially 

 note : — 



1. Fish-like shape, with dorso-ventral coloration. 



2. Loss of hair and external ears and formation of 

 " blubber." 



3. Fore-limbs formed into fins, hind-limbs lost and tail 

 forming a fin. 



4. Homodont dentition (fish diet). 



5. Modification of nostrils to form vertical blow-hole and 

 prolongation of larynx. 



6. Retia mirabilia. 



7. Loss of salivary and lacrymal glands. 



Aquatic Adaptation. 



A large number of Mammalia frequent the water either tempor- 

 arily or permanently, and the degree of aquatic habit marks the degree 

 of adaptation. We may cite the following — hippopotanms, water- 

 voles, the yapock {Ckironectes), river-shrew {Potamoga/e), otter, sea- 

 otter, walrus, sea-lions and seals, manatee and dugong, whales, porpoises 

 and dolphins. These may be studied from this point of view in the 

 following order : — 



