254 RESPIRATION. 



by placing a very little naked Gasteropod in a watch-glass of 

 sea-water under the microscope, when, on looking atten- 

 tively, you will see the water flowing in a rapid even stream 

 over the body and along the tentacula, always in one and the 

 same direction ; and a little experience in the use of the 

 glass will soon enable you to discover the minute cilia by 

 which this motion is produced. If you have not by you a 

 living mollusk small enough for the experiment, you may cut 

 off a portion from the branchiae of any species you can most 

 easily procure no matter to what class it may belong and 

 you will find the detached piece exhibiting the same pheno- 

 mena, and even swimming about in the current it has itself 

 created ; * only this you must remember, that if the animal 

 experimented upon is a marine one, sea-water must be used, 

 for the action of the cilia and impulsion of the fluid are 

 instantly stopped by putting the parts into fresh water.f 

 The important discovery of these ciliary motions enables us 

 to explain satisfactorily some appearances which puzzled the 

 earlier anatomists. Thus Carus observed that the respira- 

 tory current of water that flows in upon the bivalves is not 

 intermittent, as in almost all other animals, but uninter- 

 rupted, so that these animals, when not too deeply immersed, 

 form an eddy on the surface of the water. Now as such a 

 current cannot be caused by the alternate opening and shut- 

 ting of the shell, Carus was induced to conjecture that it 

 " must depend on a very peculiar mechanism, which consists 

 chiefly in the muscularity of the cloak, but partly also in the 

 mobility of the gills themselves, and may be compared to the 

 mechanism of certain bellows, which produce an uninter- 

 rupted current of air by means of double bags." J Blainville, 

 on the contrary, supposed that the triangular labial appen- 



out, however, having detected the cilia, says " Upon examining that part 

 of the Mussel which is called the beard, I not only found it of a wonderful 

 make, but the motion I saw in the small component parts of it was so in- 

 credibly great, that I could not be satisfied with the spectacle ; and it is 

 not in the mind of man to conceive all the motions which I beheld within 

 the compass of a grain of sand." Select Works, i. 77. I agree with the 

 fine old Dutchman ; and the impression of the spectacle, as I saw it in the 

 microscope of my friend Mr. Bowerbank, only deepens with reflection. 



* The soft gelatinous substance which supports the cilia on the branchiae 

 of the mollusca is readily detached in flakes by compression or friction. 

 These little detached flakes move about, agitating their cilia, like living 

 Infusoria ; and Miiller has described them as species of his genera Trichoda 

 and Leucophra. DUJARDIN. Hist, des Lfus. p. 147 and 677. 



f Sharpey in Edinb. Med. and Surg. Journ. xxxiv. 118, &c. Edinb.Journ. 

 Nat. and Geogr. Sc. ii. 334. Cyclop. Anat. and Phys. i. 619, &c. Edinb. 

 New Phil. Journ. ix. 383. 



J Comp. Anat. trans, ii. 148. 



