LINNEAN SOCIETY OF LONDON. T5 



exceedingly simple ; and consequently I thought it desirable 

 to start with this. In Carcinus mcenas, the common green 

 Shore-Crab, I examined its movements as they would appear 

 under water, the body being supported by the dense medium, in 

 which, of course, it normally lives, and where its movements 

 would be quite diflferent, or comparatively different, from those 

 which it would exhibit if walking upon dry sand. In this 

 decapod — so called, not because it uses ten limbs for the purpose 

 of walking, but because it has ten Imibs borne upon the walking 

 region — the anterior pair, the greater nippers or chelce, in this, 

 as in most decapods, are not used for purposes of locomotion 

 but for offence and defence, and for holding and primarily 

 tearing its prey. If the accompanying diagram (p. i6) be taken 

 roughly to represent the outline of the carapace of this crab — 

 the shield which protects its general body — it will be seen that 

 the anterior portion is provided with a series of spines more or 

 less defensive in their character, and in front is the excavation in 

 which are lodged the stalked-eyes, and between them a little 

 promontory. On each side a stalked-eye stands out, and between 

 these are the short antennae and the still shorter and more com- 

 plicated autennules. On each side are the large chelae. 



Now for the actual limbs which are concerned in locomotion. 

 There are four pairs, and, as in most of these crab forms, 

 we find two pairs — that is, four limbs — constantly touching the 

 ground, and two pairs, or four limbs, raised off the ground ; 

 those which are in action and upon the ground alternating with 

 those which are out of action and off the ground. Now, suppo- 

 sing I represent this crab crawling towards the left (and, as a 

 rule, they rather prefer crawling towards the left than in any other 

 direction, though they can move in any) : in this case the first 

 and third pairs of limbs I may represent as having just been 

 brought in contact with the ground, the left limbs being extended, 

 the right flexed. By the flexion of the extended limbs and 

 extension of the flexed, the crab will pull and push its body 

 towards the left. The second and fourth pairs at the same time 

 being raised ofl" the ground will be passing into the position for 

 action, so that when half a pace has been made all the limbs 

 will be equally flexed. "When the first and third pairs have com- 

 pleted their work they are raised, at the same time the second 

 aud fourth pairs are brought in contact with the ground. 

 During locomotion under water the body of the crab is well raised 

 off the ground and incKned upwards at an angle of about 45°, so 

 that the sense-organs are brought into a favourable position for 

 the better performance of their function. 



Now there are two distinct advantages from this particular 

 mechanism. We find, namely, that the crab always rests on the 

 firmest possible basis — it rests upon four limbs. Again, whilst two 

 are dragging, two on the opposite side (exactly corresponding) 

 are pushing the body in the same direction, and the others 

 are in exactly opposite phases. The result of this is that, if, for 



