LEPTON. 197 



shell downward, by three or four threads, which are so 

 fine that they cannot be seen by the naked eye ; and 

 even by the aid of a magnifier they can only be observed 

 when the light falls upon them at certain angles. When 

 the animal withdraws into its shell, the latter can be 

 completely closed; but usually the valves are kept a 

 little open, and the mantle protruded beyond them. 

 Mr. Clark kept a specimen alive for thirty-four days in 

 a glass of sea- water, changed daily, and it was seemingly 

 as vigorous all that time as when first placed in captivity. 

 He states that when it was in motion and had made a 

 step, the long tentacular filament curled inwards, like 

 the fore-finger of an open hand bent down to the palm, 

 but immediately afterwards it resumed a straight posi- 

 tion to await another step ; all the other filaments were 

 either retractile or contractile, separately or together. 

 The foot can form a disk at each end. According to his 

 account, a glutinous red filamentous matter is copiously 

 poured out by the byssal gland. But I would venture 

 to surmise that this phenomenon may be connected 

 with the reproductive functions. Mr. Clark goes on to 

 sav, " The animal is vivacious, and allowed itself to be 

 examined many times daily; it marched with quick- 

 ness, but I only once saw it progressing in a vertical 

 position ; the usual posture of the shell is to rest on one 

 of the disks, which is frequently changed for the other ; 

 the adductors did not appear to allow of a greater open- 

 ing of the valves than the ordinary extent. The animal, 

 when placed at the bottom of a glass, always crawled 

 up and moored itself by a filament at the side ; some- 

 times, however, it slipped its moorings and floated free 

 on the surface of the water with the umbones down- 

 wards, and after an interval refixed itself by spinning a 

 byssal thread." He subsequently succeeded in obtain- 



