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 Cefacea 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 (1) 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 431 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, bind-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 Mammatia frequent the water either tempor- 
arily or permanently, and the degree of aquatic habit marks the degree 
of adaptation. We may cite the following—hippopotamus, water- 
voles, the yapock (Chéronectes), river-shrew (Potamogale), 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 :— 
