154 THE BIOLOGY OF THE SEA-SHORE 



of scansorial adaptation are the most numerous ; the uneven 

 nature of the substratum is largely responsible for this. 

 Many others again spend their time climbing among sea- 

 weeds where they find their food or to which they cling, at 

 least, in order to resist dislodgment by water movements. 

 Biologically, such forms correspond to the terrestrial wall 

 and rock climbers or to arboreal forms. Adaptations to 

 climbing over rocks and weeds are well seen among the 

 group of Decapod Crustacea sometimes referred to as the 

 Reptantia, which includes the lobsters, hermit crabs and their 

 allies and the true crabs. In contrast to the swimming forms 

 or Natantia (shrimps and prawns) the Reptant Crustacea 

 tend to have their bodies dorsally flattened ; moreover, the 

 thoracic limbs are much more strongly developed. The 

 lobsters are more or less intermediate forms ; those which 

 show the greatest degree of adaptation to the clambering 

 habit are the crabs. In the crabs the body is, of course, 

 very much broadened, and with this broadening, first obvious 

 in the hermit lobster Galathea, as Newbigin (1901) points 

 out, the insertion of the legs moves outwards " so that the 

 body becomes more definitely adapted to the creeping 

 habit." Moreover, in typical clambering forms like the 

 crabs, of the five pairs of thoracic legs only the first pair is 

 chelate, the remaining four pairs being devoted exclusively 

 to supporting the body. But it is chiefly in the long 

 straddling legs with their incurved claws that the crab shows 

 itself adapted to clambering and to taking a grip of flat 

 stones ; it is not too much to say that there is something 

 resembHng an ectoparasite about the appearance of a crab. 

 Stebbing (1893) quotes the following description by Miss 

 J. M. Arms of the method of locomotion of the common 

 American species of crab Cancer irroratus : " The legs of 

 one side are used to push with and those of the other to 

 pull with, when the crab is in motion. Those of the same 

 side do not, however, all move together, but alternately, 

 so that there is no halting in their gait ; some of the legs 

 are always in the act of taking new steps and by shoving 

 and pulling in unison a continuous motion is kept up." 



