IrlO' Natural History of Molluscous Animals : — 



cretory orifice, or that from which the general current 

 already mentioned is observed to come out. That portion 

 of the powder which remains outside the gill is carried 

 along its surface in straight lines from the base to the margin, 

 along which it then advances onwards towards the fore part 

 of the animal. As the spaces between the layers of all the 

 gills terminate directly or indirectly at the excretory orifice, 

 it is easily conceivable that the water, penetrating by the 

 entire surface of these organs, may, by their concentrated 

 effect, give rise to the powerful current which is observed to 

 come out from the animal. 



" On examining a portion of the gills with a powerful 

 lens, I perceived that it was beset with minute cilia, which 

 are evidently instrumental in producing the different currents. 

 Most of them are ranged along the anterior and posterior 

 margin of each of the vessels composing the gills, in two 

 sets : one nearer the surface, consisting of longer and more 

 opaque cilia; the other close to the first, but a little deeper, 

 in which they are shorter and nearly transparent. Both sets 

 are in constant motion, but of this it is difficult to convey a 

 correct idea by description. The more opaque cilia, or those 

 of the exterior range, appear and disappear by turns, as if 

 they either were alternately pushed out and retracted, or 

 were continually changing from a horizontal to a vertical 

 direction. The motion of the other set appears to consist 

 in a succession of undulations, which proceed in a uniform 

 manner along the margin of the vessel from one end to the 

 other. It resembles a good deal the apparent progression 

 of the turns of a spiral when it revolves on its axis, and 

 might very easily be mistaken for the circulation of a fluid in 

 the interior of a canal, more particularly as the course of the 

 undulations is different on the two edges of the vessel, being 

 directed on the one towards the margin of the gill, and on 

 the other towards the base. But, besides that the undu- 

 lations continue to go on for some time in small pieces cut 

 off from the gill, which is inconsistent with the progression 

 of a fluid in a canal, the cilia are easily distinguished when 

 the undulatory motion has become languid- When it has 

 entirely stopped, they remain in contact with each other, so 

 as to present the appearance of a membrane attached to the 

 edge of the vessel. 



" It is very remarkable, that, when the gill is immersed in 

 fresh water, both the currents and the motion of the cilia are 

 almost instantaneously stopped." * 



* On a peculiar Motion excited in Fluids by the Surfaces of certain 

 Animals ; by William Sharpey, M.D. Edin. Med. and Surg. Journaly vol. 

 xxxiv. p. 118, &c. 



