1 8 PEOCEEDIKGS OF THE 



him to realize at once the various phases of the movement, and 

 thus to select from those phases that one which he may most 

 require for pictorial representation. 



The next form which I will speak' about is one of these 

 brachyurous Crustacea, in which the rhythm is extremely different 

 for certain evident reasons. It is a crab whose carapace is very 

 much smaller than the one just described, and is more or less> 

 heart-shaped. It is the crab known as Inachus. 



This crab, unlike most others, never uses the first pair of 

 limbs for the purpose of walking. These are much larger than 

 the others, but in locomotion they are extended off the ground^ 

 almost horizontally, outward. There is the usual number of six 

 joints — the common number possessed by crabs. These limbs, 

 which are not used for locomotion, are more abundantly covered 

 with the little hooked hairs which the external surface of this 

 crab naturally bears, and to these hooked hairs the crab fixes (as 

 all crabs do when they have these hairs) little pieces of foreign 

 bodies, which it attaches for the purpose of concealment. This 

 particular species commonly selects little pieces of sponge, which 

 it fastens upon its large limbs, and these subsequently grow and 

 eucase the limb, so that it becomes a warty or gouty-looking limb, 

 covered with pieces of sponge intentionally affixed by the crab, 

 and afterwards, as it were, taking root and growing there. Now 

 for the remaining three pairs. They, of course, have to move in 

 a perfectly difi"erent rhythm from what would be the case if the 

 crab had four pairs of effective limbs. Supposing the second of 

 these on the left-hand side to have just come into action, we 

 should have it extended, in contact with the ground, and pulling. 

 Instead of, as in the case of Carcinus mcenas, the corresponding 

 limbs on the opposite side pushing — it is not so, it could not be, 

 because there would be no true continuous movement if that were 

 the case, — the limb or limbs in action on the right side are not 

 the corresponding limbs to those in action on the left, but the 

 alternate oues. The creature, therefore, is always resting on a 

 tripod instead of on four limbs, with one limb on one side pulling 

 and two on the opposite side pushing. 



There is another form of some interest. It is the common 

 Stone Crab {Litliodes) of the northern coast. Instead of being 

 reduced to a hexapod, as in Inachus, by the non-use of the 

 anterior pair of limbs for purposes of locomotion (they being 

 simply raised off the ground as a sort of balancing pole), the 

 MnA pair of limbs, greatly reduced in size in Lithodes, are used 

 to clean out the branchial chamber and adjacent parts, and thus, 

 by suppression for locomotive purposes of, the liind pair of limbs, 

 the form is reduced to a hexapod, and so has precisely the same 

 hexapod character of movement as is observable in Inachus. 



"We may now consider those cases in which the locomotive 

 limbs are reduced to foxir. This reduction, one would naturally 

 think, would lead to considerable complication, on the suppo- 

 sition that if more than one limb were raised at a time 



