. Mr. J. Alder on the Animal of Kellia rubra. 51 



long siphons (Mi/a, Lutraria, &c), the branchiae, being situated 

 at a great distance from the apertures, may require from time 

 to time the assistance of muscular contraction for a thorough 

 cleansing out of the branchial cavity, and in this case the water 

 will be discharged out of both siphons from the stronger force 

 overcoming the action of the cilia*. 



Mr. Clark takes some pains to prove that the water does not 

 make a circuit through the intestines, which position, being un- 

 disputed and apparently unconnected with the argument, I should 

 not have noticed but for the conclusion drawn from it ; which is, 

 " that the water therefore " (on account of not passing through 

 the intestine?) "for the branchiae and sustentation must pass 

 into the great branchial cavity, and issue therefrom by both the 

 ducts at which it entered" How is this ? The conclusion appears 

 to be a non sequitur: but possibly I may misunderstand the 

 meaning of the paragraph, though I have read it over carefully 

 more than once. 



With respect to my statement of having seen, under the mi- 

 croscope, a continuous current of water flowing into the anterior 

 tube of Kellia rubra, Mr. Clark observes, u All must admit this 

 fact : as the fold is a part of the open mantle, no microscope is 

 here required, as in every open-mantled bivalve of adequate size 

 this action is instantly made apparent by a common lens, and is 

 the invariable result of the animal opening its valves" In Mr. 

 Clark's former letter he says, " No currents, at least branchial 

 ones, enter therein or issue therefrom ; it is a fold merely sub- 

 servient to locomotion/' The flow of a continuous current into 

 this tube-like fold is now treated as an admitted fact, requiring 

 no microscope for its demonstration ; — but it is attributed to the 

 opening of the valves. It may be necessary therefore to state that 

 the operation goes on when the valves are perfectly at rest, and 

 cannot in that case be produced by their means. That I could 

 see a current passing out at the posterior aperture is however to 

 Mr. Clark a matter of the <f gravest difficulty," only to be got 

 over by supposing that I was deceived by the u aberration and 

 well-known great deceptions involved in the use of high micro- 

 scopic powers." It will be a satisfactory answer to this to state 

 that I was able to see it with the lowest power of my microscope, 

 where there could be no aberration. The advantage of a micro- 

 scope over a pocket-lens in this case is the greater facility it 

 affords in managing the light, which requires to be transmitted 



* The internal surface of these siphons is usually (perhaps always) covered 

 with vibratile cilia, more minute than those of the branchiae, but acting in 

 conjunction with them in producing the currents. Mr. Cocks informs me that 

 he can see the cilia inside the anterior tube of Kellia suborbicularis, with a 

 lens of ^-inch focus. 



4* 



