38 



G. 0. SAKS. 



[No. 1. 



mical manner, so as but rarely to get stopped up. Thus all the 

 legs move in a pendulous manner, swung at regular intervals 

 backwards and forwards, the former movement being however 

 somewhat more rapid than the latter. But this pendulous mo- 

 tion is not simultaneous in all the legs ; it takes place suc- 

 cessively, a little later in each succeeding pair, and in such 

 manner, that at the moment when the first pair is moved back- 

 ward, the last pair makes the contrary movement forward. 

 Thus a very peculiar and graceful undulatory aspect is given 

 to the legs when moving, the undulations, it would appear, 

 proceeding equably, in a posterior-anterior direction. The muscles 

 performing these movements of the legs, have partly been 

 described above. 



B. Sensation. 



At least three different specialized sensations should be attri- 

 buted to the animal, viz., the sense of tact, that of smell, and that 

 of sight, each having apparently a distinct organ of manifestation. 



The sense of tact cannot properly, in my judgment, be attri- 

 buted to any of the organs constantly covered by the shell, only 

 the parts of the body that admit of being exserted from the 

 edges of the valves being adapted for such a function. Moreover, 

 it is evident that the tactile organ must be sought for at the 

 anterior part of the body and not at the posterior, the former 

 being generally that which first touches an object during the 

 normal motion # of the animal. As above stated, the extremity 

 of the head, together with the antennæ, are the only parts 

 that admit of being exserted in front of the shell, and of 

 these parts the peculiar recurved spines affixed to the distal 

 portion of the scape of the antennæ and to the proximal joints of 

 the upper branch of the same organs, are at first invariably 

 found projecting beyond the edges of the valves, when the head 

 is about to be thrown out after being withdrawn into the shell. 

 Hence I would attribute to the antennæ, besides their locomotory 

 function, also that of producing tactile sensations, the latter 



