36 Mr. Jeffreys on British Mollusca. 



nute water-insect had found its way unbidden into the globe, and 

 was thus iUustrating, Hke some learned lecturer to his sleeping au- 

 dience, the laws of planetary motions. But no such lecturer was 

 there ; yet, as the revolution brought two of the audience closer under 

 inspection, I observed their siphons to be curved exactly in the oppo- 

 site direction to the line of motion. Here was a solution, at once, of 

 the nymph4ike arpoftiXos, which was evidently due to the recoil 

 consequent upon the circulation and expulsion of the water through 

 the siphons. The fortuitous position of the two individuals and the 

 combined action of their expulsive tubes may not occur again ; but 

 the whole incident was so interesting and remarkable, that I could 

 not help recording it. The fry are growing rapidly ; and I opine the 

 amount of exercise they indulge in is conducive to their health. I 

 have observed the Eulima distort a, Rissoa parva and cingillus, as 

 well as the Odostomice and Jeffreysice, ascend to the edge of a basin 

 and creep along the under-surface of the water, in the same manner 

 as the LymnceadcB. But it is singular that bivalves should imitate 

 their less unwieldy moUuscan brethren in this seemingly unsuitable 

 mode of progression." In another letter he says, " Sometimes a 

 single individual will suspend itself to a little bit of the stem of the 

 Lemna, and whirl quite alone for hours, even rapidly — say fifteen to 

 twenty revolutions in a minute. The quantity of weed in the globe 

 was very small." And in a subsequent letter he goes on to say, 

 *' The young are far more active than the parents. I do not perceive 

 their siphons to be ever exserted, while this is almost constantly the 

 habit of the older ones. They all continue to climb the glass globe, 

 and rather more so in the evening, probably preferring to roam in 

 the dark. I have had a fresh supply of about half-a-dozen, which, 

 soon after being immersed, began an inspection of their new domain, 

 and continued for a day or two more restless than the others. On 

 climbing the glass, the front' margin of the valves is applied to it, 

 and at the same time both the foot and the siphons are exserted. 

 The foot being extended to its full length, its extremity is cautiously 

 pressed against the glass, and after a short pause the upward move- 

 ment of the body commences, which is the work of a second of time ; 

 then another short pause, after which the front margin of the valves 

 and the point of the foot are again applied cautiously to the glass, 

 and the foot is again protruded to repeat the same process. When 

 the edge of the water is reached, the pauses are longer, and it is 

 necessary for the creature to be doubly cautious, for here is the point 

 of greatest difficulty. However, the foot is conveyed horizontally 

 along the under surface of the water, which appears to recede par- 

 tially from it. On examining it with a lens, the foot is distinctly 

 seen to have an undulating action on the surface, as well as an irre- 

 gular and imperfect contraction and elongation along its whole extent ; 

 but it is never quite retracted, excepting when its base and the front 

 margin of the valves are in contact with some floating weed which is 

 capable of supporting the whole. Thus this elegant shell traverses 

 the still surface. But it is most curious to see it descending the 

 thread-like stems of the Lemna, or some assemblage of these delicate 



