Currents in Pholas and Mya. 373 



muddy bottom in full action. The mud lies closely packed 

 against the walls of the tubes, so that nothing is to be seen but 

 the internal surface of the expanded lips of the siphonal orifices 

 fringed with numerous tentacles. When it happens that the sur- 

 face of the water is only a little above these orifices, a strong cur- 

 rent can be distinctly seen to boil up from the anal siphon, and 

 another, with a constant, steady flow, to set into the branchial 

 one. These currents were quite visible to the naked eye without 

 the aid of a glass, so long as the mollusk remained undisturbed. 

 We watched one individual for nearly a quarter of an hour, and 

 no interruption of them took place ; and it was not until the 

 siphon was touched that the tubes were withdrawn and the 

 currents ceased to play. But the siphon soon made its appear- 

 ance again at the surface, and the orifices once more expanding, 

 the currents commenced to play as strongly as ever. On exami- 

 ning the animal thus in its natural position, it is impossible to 

 doubt that the currents are connected. If not, whence comes 

 the water that is constantly pouring out of the anal orifice ? 

 None can pass down externally to the pedal opening while the 

 tubes are in action, for at this time they completely fill the 

 channel they occupy, their walls being in close contact with 

 the sides of the burrow; and the mud, though it lay loosely 

 about and against the siphons, was not in the least disturbed, as 

 it must have been had a current passed down outside the tubes. 

 Yet to satisfy Mr. Clark's theory such a current must have 

 existed. 



On removing these animals from their concealed abodes, and 

 placing them in a vessel of fresh sea-water, the two siphonal cur- 

 rents were generally found in action when the individuals, were 

 undisturbed. And further, on placing the shell with its back 

 downwards and the pedal gape raised above the surface of the 

 water, these currents still continued to play ; the excurrent and 

 incurrent being as distinctly observed as before. 



With Pholas we tried another experiment, which would seem 

 conclusive with regard to the connexion of the siphonal currents. 

 A specimen of Pholas crispata was placed in a shallow vessel of 

 sea-water, and, as usual, the currents soon commenced to play. 

 The nosle of a blowpipe charged with sea-water stained with in- 

 digo was then placed at the inhalant orifice, and immediately a 

 considerable quantity of the coloured medium was drawn into 

 the animal. Watching carefully the result, we had soon the 

 satisfaction of beholding a blue-stained stream issue from the 

 exhalant orifice. This experiment succeeded three or four times, 

 notwithstanding the caution of the animal, which generally, on 

 receiving the contents of the blowpipe, immediately closed the 

 inhalant orifice, and then gradually contracting itself, with a 



