THE MOVEMENTS OF SHORE ANIMALS 155 



This crawling by means of jointed appendages can be readily 

 imitated in the following way. " Cross the two wrists side 

 by side, placing the fingers down on a level table ; bind 

 the wrists by an elastic band, hold them well up from the 

 table, so as to show the fingers. Then let one set crawl 

 while the other pushes, so as to keep up a continuous motion 

 sidewise without assistance from the arms. The terminal 

 sections of the legs show wear only on the points where these 

 are inserted in the ground." In tropical crabs of the genus 

 Grapsus the art of moving over a broken surface has appa- 

 rently reached a high degree of development. Speaking of 

 the forms G. grapsus and G. strigostis, Alcock (1902) says that 

 they are exceedingly difficult to catch, running very fast and 

 dodging and doubling like a hare — " their bodies are so thin 

 and flat that they can cling to any surface, however steep ; 

 they keep such constant watch that one can never surprise 

 them, and even when you think you have succeeded in 

 cutting off their every chance of escape, they hurl themselves 

 headlong into the sea and disappear." 



Amphipods, like Gaminarus, which climb over weeds 

 have their bodies laterally flattened, and when clambering 

 over a weed with fine branches the plant is grasped between 

 the appendages, the lateral extensions of the body-covering 

 (pleura) aiding the animal in keeping its balance. Both 

 Amphipods and Isopods have the last joint or claw of the 

 legs incurved and the limbs show the same general 

 adaptation to clinging as do those of crabs. In Ltgia the 

 shape (flattened dorso-ventrally) of the body is adapted to 

 climbing over rock and stones rather than over weeds. 



The Pycnogonids are found climbing over seaweeds and 

 zoophytes, a habit to which they are admirably adapted by 

 their long slender appendages each of which ends in a sharp 

 recurved claw. The movements, however, are slow and 

 deUberate, " manifestly not adapted to capture or to kill 

 a living prey " (D'Arcy Thompson, 1909) 



It is not surprising that the Arthropods should furnish 

 such a large number of ectoparasitic forms when the nature 

 of their appendages fits them so well for clinging. It is 



