14 SPH^RIIDiE. 



amount of exercise they indulge in is conducive to their 

 health. I have observed the Eulima distorta, Rissoa 

 parva and cingillus, as well as the Odostomice and Jef- 

 freysice, ascend to the edge of a basin and creep along 

 the under- surface of the water^ in the same manner as 

 the Jjymnceadce. 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 sus- 

 pend itself to a little bit of the stem of a Lemna, and whirl 

 quite alone for hours^ even rapidly — say fifteen to twenty 

 revolutions in a minute/^ 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 movement 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 



