256 Dr. T. Williams on the Mechanism of Aquatic 



out are of a deep dark green colour [c) ; this colour is concen- 

 trated in the nucleus. Those which are within the limitary 

 membrane of the caecum are less darkly tinged (rf). But it is 

 evident that the latter cells circumscribe contents which are the 

 derivative products of the former. The cells situated in the cir- 

 cumference of the vesicles are more dark than those placed in the 

 centre. This gradation of colouring is expressive of the stages 

 through which the secreted product passes. By this coloured 

 substance this gland is strikingly distinguished from that of the 

 " muciparous gland." The follicles differ in figure from those of 

 the latter, but the cells do not ; they are distinguished only by 

 the colour of the material by which they are filled. 



Arbitrating as mere physiologists, it would be quite impossible 

 to say why one of these glands should be a kidney and why the 

 other should not. It cannot be proved by the secreted product 

 of this gland that it is one really designed to furnish a '' coloured 

 fluid." This inference is founded simply upon the colour of the 

 cell- machinery by which this product is elaborated — not upon 

 the colour of the finished excretion. 



The author proposes for the present to leave this question in 

 an unsettled attitude; but he will venture to state that neither 

 of these glands is the source of the " mucus " or " mucosity " 

 for which the Gasteropod Mollusks are remarkable. This " mu- 

 cosity " is really supplied by a totally different machinery. It is 

 poured forth by follicles which in all Gasteropod and Nudibran- 

 chiate Mollusks are more or less thickly strewn over the mucous 

 and cutaneous surfaces. These follicles will be more minutely 

 described in speaking of the respiratory organs of the Nudi- 

 branchiate Mollusks. 



EXPLANATION OF PLATES V. and XI. 



Plate V. 



Fig. 1. Animal oi Littorina Uttorea, removed from its shell, having the 

 branchial chamber laid open, — the roof being everted in order to 

 show its under or internal surface, e, e, roof; d, colour-gland of 

 the author, " double "-gill of Dr. Sharpey and other naturalists ; 



a, a, branchia, consisting of many hundreds of parallel leaves j 



b, a gland of the mucosity, supposed to have a renal function ; 



c, extremity of the intestinum rectum ; g, a second gland whose 

 function is not determined ; j, duct of the generative organ ; 

 i, floor of the branchial chamber ; A, mantle ; h, abdominal por- 

 tion of the body. 



Fig. 2. Animal of Buccinum undatum, removed from its shell, and showing 

 the roof of the respiratory chamber everted : /, siphon ; 6, the so- 

 called double-gill, the colour-gland of the author ; a, a', branchia, 

 consisting of many hundreds of parallel leaves ; d, duct leading 

 from the colour-gland and terminating near the rectum, e ; c, gland 

 of the mucosity, supposed to have a renal function. 



