chap, x.] MOVEMENTS OF CHORD AT A. 381 



greatly enlarged, and form distinct epipodia, and 

 these, either independently or in conjunction with the 

 median part of the foot, become converted into 

 powerful muscular fins. In the Cephalopoda the 

 epipodia form a funnel, through which the water of 

 respiration is expelled to the exterior ; this expulsion 

 of the water forwards results in a backward movement 

 of the animal. In the Tetrabranchiata (Nautilus) the 

 edges of the epipodia 

 are not, as in th 

 Dibranchiata, fused 

 with one another, but 

 merely folded over. 



Among the Chor- 

 clata locomotion is 



effected by swimming, r te- 161.-Cfcrroewta cymlium. 



i n m m* r> a- rT>PPr>in o- m > Proboscis ; t, tentacles : /, foot ; d, disc ; 

 jumping, eeping, s, shell; g, branchiae. 



walking, or flying, and 



all these activities are presented by marine as well as 



by terrestrial forms. 



Swimming organs have the form of more or less 

 broad plates, which may or may not be supported by 

 bone. The simplest cases are presented by the mere 

 flattening or expansion of an organ ; this, for example, 

 obtains in the tadpole, the newt, or the insectivorous 

 form Potamogale velox, where the tail is flattened 

 from side to side and forms a powerful locomotor 

 organ. In the Cetacea, on the other hand, which 

 require to come repeatedly and rapidly to the surface 

 of the water, the tail is flattened from above down- 

 wards. 



In more complicated cases, as in many fishes, the 

 tail, which is here also the most important organ of 

 locomotion, and has a screw-like movement, is aided 

 by the caudal fin when that is well developed ; the 

 paired lateral fins are in most cases rather organs of 

 support and direction than of locomotion ; but in some 



