550 CHORD AT A. 



1. 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 {Intra) 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 Sirenia 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 

 niirabilia^ to allow of " holding the breath" under water. 



The Cetacea are further adapted than the Sirenia. 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 vertebrce fuse into one mass. Salivary glands for 

 moistening food tend to disappear. There are special adaptations to a 

 fish diet (homodont dentition), as in Odojitoceti, and to a plankton diet 

 (pelagic animals), as in Mystacoceti. 



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 [Sirenia) 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. 



