180 ON CKABS WHICH CLING. 



the shell or carapace and on the basal part o£ the claws are of 

 great importance as filters preventing large solid particles 

 from being swept on by the strong current of water which sets 

 into the gill cavity. 



It has already been said that the ordinary crabs are 

 descendants of some form like Dromia and there are too many 

 differences of detail, notably the form and division of the 

 telson or last part of the abdomen, between Dromia and 

 Galathea^ for us to suppose the former is a descendant of 

 the latter. They are perhaps related in a degree analogous 

 to that of first cousins once or twice removed ! 



None the less the true crabs show us several of the 

 developments noticeable in Gralathea carried very much 

 farther. The shortening of the body is very conspicuous, 

 breadth even exceeding length in several members of the 

 group. The habit of walking sideways is decidedly prevalent 

 and there is an increased power of variation of the direction of 

 locomotion. Both these features are particularly well seen 

 in our edible crab, Cancer pay urns. This type has the back 

 much broader than it is long and the legs can be to a large 

 extent tucked under ; the crab's back in fact is produced on 

 either side into a blunt sort of chisel or crowbar-end, with 

 which the crab is able to force its way under stones and through 

 through masses of weed, &c., where more spreading limbs 

 woidd get hopelessly mixed. But this is only an aside ; the 

 point to which I should like to draw attention is that the 

 shortening of the body has helped to increase the centralisation 

 of nervous control still more than in Galathea, and this 

 structural evolution has made it possible for what I can best 

 name " mental evolution " to take a great stride. The crabs 

 are far ahead of the lobster tribe in the variations of action 

 and in the power of modifying instincts or habits as a result 

 of chance experience. Indeed some of their habits, liomanes 

 mentions an action of a sand crab to which I must refer, can 

 hardly be understood without supposing that the animal 

 had reached an even higher stage of mental development. 

 This particular individu;il was incommoded by a stone which 

 fell into his burrow, whereupon he not only cleared it out, 

 but also removed to a safe distance all stones which were 

 near enough to be in danger of playing him the same trick. 

 In many of the crabs, mating habits have also been developed 

 to a considerable extent, while among the lobsters courtship 

 has not got beyond the fighting stage, so that here again crabs 

 are on a higher level than lobsters. Another resemblance 

 between Galathea and many crabs, in addition to nervous 



