326 OBSERVATIONS ON MOLLUSKS. 



a few species : the Cyclas calyculata ft*, and the 

 Pisidium obtusale, shew it in perfection ; whereas I 

 never noticed it in the P. pusillum, or the P. pulchel- 

 lum,-\ though both these will freely ascend the sides 

 of the vessel, and the latter especially is at times 

 very active and lively in other respects. J 



* Monograph? p. 298, tab. xix. f. 1. 



f Id. p. 306, tab. xxi. f. 23. 



J The above remarks on the habits of these bivalves are ex- 

 tracted from my Monograph ; at the time of publishing which, I 

 was not aware of any one who had attempted to explain this 

 singular property, possessed by certain mollusks, of walking in an 

 inverted position immediately beneath or against the surface of the 

 water. Since then I have met with a passage in the Annales fits 

 Sciences Naturelles, (2nd series, torn. xix. pp. 309 10,) in which 

 it is alluded to by M. Quatrefages, who explains it by referring it 

 to ciliary action. As his remarks are interesting, and enter more 

 into the details of the phenomenon than what are given above, I 

 will here translate the passage in question, though perhaps the 

 phenomenon may require some further explanation than that which 

 he offers. The passage occurs in a memoir on the Eolidina para- 

 doxum, a new genus of Nudibranchiate mollusks, in which he finds 

 vibratory cilia existing on every part of the surface of the body. 

 In reference to their supposed occurrence in other gasteropoda, he 

 says : 



" The peculiarity that I have just noticed, seems to me to serve 

 to account for a fact, which has at different times attracted the 

 attention of malacologists, and of which I have nowhere seen any 

 very satisfactory explanation. It is known that almost all the 

 gasteropods can walk, as it were, on the surface of a liquid, turn- 

 ing themselves upon their back. In this position, the lower sur- 

 face of the foot coincides nearly with the superficial stratum of the 

 liquid, and it is seen contracting itself and bending itself in 

 different directions, as if it took its resting-place upon the stratum 

 of air which it just grazes : the animal advances, though very 

 slowly, without employing any other apparent means of locomo- 



