1/8 ON CRABS WHICH CLIXG. 



also notice that the great claws are extremely long so that 

 they may reach out of the crevice ; the body on the other 

 hand is comparatively short from front to back, and this 

 in part assists hy enabling the animal to use nooks and 

 crannies that would otherwise not be deep enough. The 

 shortness of the body has however another meaning we 

 shall discuss later. When Galathea is waiting in this way 

 " at the receipt of custom," the three pairs of walking legs be- 

 hind the great claws lie forwardly directed and more or less 

 parallel to one another, assisting to hold the animal in position 

 if necessary. The long antenme (the second pair) are useful 

 for feeling, but function especially in the animal's more active 

 moods. The smaller feelers (first pair) or antennules lie 

 exposed and have a characteristic flicking motion in some 

 way connected with the sedentary habit, for we find that 

 peculiarity in most of the lurking Decapods, while the more 

 primitive types have the antennules long and built more or 

 less on the pattern of the antenuie. The antennules possess a 

 row of very well developed hairs, and probably function 

 as smelling or more exactly as water-testing organs in these 

 types, whose home is no longer the open main, but a sheltered 

 nook of the sea floor, or the tidal shore, which may become too 

 foul, even for the not over sensitive fancy of a crab. 



Galathea however does not spend by any means the whole 

 of its life waiting for prey ; it will leave its sheltered nook 

 anon and crawl about the neighbourhood, very likely snipping 

 off a piece of seaweed here and there to vary its diet. It 

 is worthy of notice that the direction of motion is often at an 

 angle to the median plane, a habit which has become fixed 

 ostensibly because in this way the well-armed claws, instead of 

 the delicate sense organs of the head, would obviously be the 

 parts to come into contact with the many obstacles of a region 

 of crevices. The spines on the legs may be incidentally useful 

 as comb-teeth, helping the animal as it walks among filamen- 

 tous seaweeds. This sideways motion tells us at once that 

 there must be a more intimate correlation, a more complete 

 harmonisation, of the movements of the various walking 

 limbs than is the case among lobsters where these all usually 

 act in more or less the same way at the same time. It is 

 worthy of notice too that through this specialisation the 

 animal is able to walk fairly easily in at least two directions 

 instead of merely forwards in the direct line. We may in a 

 preliminary way connect this increased correlation, this grow- 

 ing centralisation of control, with the shortening of the body 

 already referred for discussion below. 



