166 Proceedings of the Eoyal Physical Society. 



it is only to be noticed that in the less modified claws of the 

 5th and 6th limbs the arrangement of the tactile groups is 

 not so definitely marked along the edges as in the great' 

 claw ; ^ the tufts are large and scattered irregularly over the 

 surface of the pincers. On the spikes terminating the 7th 

 and 8th limbs the arrangement is the same as in dactylopodite 

 of the 5th and 6th. 



Experimental, etc. — The effect of touching the tufts on the 

 pincers of the great claw has been already described, and 

 will be further entered into in discussing the nervous ap- 

 paratus ; suffice it here to say, that the result of .touching 

 the tufts in the lesser claws is precisely the same as in the 

 great claw. If the sensitive surfaces (see above) of the 

 meropodite or carpopodite of the great claw be touched, both 

 the small claws endeavour to seize or sweep away the off'end- 

 ing object, and the great claw is moved backwards as a 

 whole in the attempt to seize it. The ventral surfaces of 

 the coxopodites of all these limbs are sensitive, and if they 

 be touched, all the thoracic limbs, with the exception of the 

 maxillipedes, turn their points inwards, and endeavour to 

 reach the object which is attacking them. 



The aggregation of the tactile setae into tufts is interesting, 

 and, from the fact that it has not been described in any of 

 the lower Crustacea, and also from the distribution of these 

 tactile groups in Astacus, it evidently results from the thick- 

 ness of the integument. In addition to the obvious con- 

 venience of having the nerve-canals of a number of set?e 

 close together, a concentration and amplification of the tactile 

 impression is thereby secured, which would be impossible if 

 the same number of setae were scattered evenly over a larger 

 space. 



In the lobster's claw, where the integument is much 

 thicker, a still greater centralisation of the tactile setae has 

 taken place. They occur either as a single row of thickly 

 set tufts along the inner edge, sheltered by the tubercles, or 

 as a single branch at the base of the dactylopodite, all the 

 rest of the surface of the pincers being entirely free from 

 them. 



1 See Huxley, Crayfish, Fig. 46. 



