550 CHORDATA. 
I. The incidental group.—Leaving out of consideration the hippo- 
potamus, water-vole, musquash and other mammals which frequent 
water but do not show marked adaptations thereto, we have the duck- 
mole, coypu, yapock, desman, river-shrew, otter and beaver. These all 
swim actively in the water and the toes are often united with a web of 
skin which enables the limbs to act as paddles. In addition, the tail 
is usually modified. In many cases its hair is lost and it is scaly and 
flat (cf duckmole, beaver, river-shrew). They are all freshwater river 
animals, and the majority are also fossorial, living in holes, so that the 
claws remain long and powerful. They are fairly at home on land and 
retain their hair. 
2. The transition group.—The sea otter (/#/ra) carries us on to 
the second group of the walrus, sea-lion and seals. | Here the body is 
fish-like and the limbs are modified into true paddles, the front-limbs 
forming the steering paddles and the hind-limbs the motor paddles. 
The terrestrial habit is more and more forsaken. The walrus and sea- 
lion can still place the sole of their hind-limb on the ground and can 
walk clumsily. They come ashore to breed. The seal has progressed 
further. The hind-limbs are permanently bent backwards for swimming 
and the external ears have disappeared. In all this group, however, the 
hair remains as fur all over the body. 
3. The true aquatic.—The Svrenia or manatee and dugong and 
the Cetacea remain. They are fish-like in shape, the fore-limbs are 
formed into paddles and the hind-limbs have disappeared altogether 
as the motor paddle is formed by the tail. In this respect they carry 
on the adaptation of group I rather than group 2, which form their 
motor paddle from the hind-limbs. The hair is almost entirely lost and 
the pinna of the ear is lost. The claws, reduced in group 2, are lost 
here. The blood-system has networks of blood-vessels, called retia 
mirabilia, to allow of ‘‘ holding the breath” under water. 
The Cefacea are further adapted than the Sivexia. They become so 
fish-like in form that they were for a long time supposed to be fish. 
Many have the dark upper-surface and light under-surface characteristic 
of fish (dolphin, porpoise). The front-limbs are very shortened for a 
sharp quick stroke, and the phalanges are increased in number from the 
normal mammalian type. The nostrils open on the top of the head 
and in many there is a dorsal fin. A flexible neck is no longer required 
and the cervical vertebrae fuse into one mass. Salivary glands for 
moistening food tend to disappear. There are special adaptations to a 
fish diet (homodont dentition), as in Odontoceti, and to a plankton diet 
(pelagic animals), as in Mystacocetz. 
We may trace the evolution of aquatic forms from the resort of 
fossorial types to the soft ground in the neighbourhood of rivers, then 
to the acquirement of aquatic food, either fish or water-weeds. The 
river leads to the river mouth (Szvevza) and this to the open sea. The 
Pinnipedia, however, may have taken to the sea direct from a polar- 
bear-like habit. 
In all, the mammalian type has its teeth modified for the fish diet 
and the limbs and tail modified for the fish mode of locomotion, sharp 
short strokes with a large surface being the end to be attained. 
