COLOUR OF CEPIIALOrODA. 237 



another explanation, founded on a connection between the 

 vesicles and the nervous system, which he imagined he had 

 traced. The colour is correctly stated to he inherent in the 

 tissue itself; and its changes are attributed to the particular 

 structure of the vesicles, each of which, he says, has a cir- 

 cular aperture that opens and shuts, probably by means of a 

 circular muscle, the actions of which are regulated by the 

 will of the animal, through the medium of the nerves, with 

 which the vesicles are connected by means of delicate fila- 

 ments, scarcely discernible even with the aid of a microscope. 

 But the observations of Dr. Coldstream destroy this inge- 

 nious and plausible theory. Dr. Coldstream could not dis- 

 cover, by the most careful microscopic examination, the 

 slightest trace of any nervous threads in connection with the 

 vesicles; and he proved, which seems incompatible with their 

 dependence on their nerves, that the vesicles possessed mo- 

 tion in pieces of the mucous coat which had been removed 

 entirely from the body : nor could he succeed in discovering 

 any opening in them, such as San Giovanni asserts they 

 have, even during their greatest dilatation, and under the 

 most favourable circumstances. " That I might ascertain," 

 to use Dr. Coldstream's own words, " whether or not the 

 motions of the spots were now (after ajiparent death had 

 taken place) carried on by the influence of external agents, 

 independently of any nervous power emanating from the 

 animal itself, I cut, from a part of the mantle where the 

 contractions and dilatations were very strong, a piece of the 

 membrane or layer containing the spots, about two-tenths 

 of an inch square ; this I separated completely from the 

 animal, and placed it in a watch-glass immersed in sea-water 

 in another vessel. To my astonishment, I saw that the spots 

 in the separated portion continued in as lively motion as 

 when connected with the animal. No change, either in the 

 velocity or extent of their motions, could be perceived. 

 Some spots, just on the edges of the sepai'ated piece, seemed 

 to have been half cut through by the scissors with which I 

 removed it : such did not contract ; but all the others in the 

 piece moved in the very same manner as before. 



" I now removed the watch-glass, containing the separated 

 portion, to the stage of a microscope, and examined the 

 spots with powers of 100 and 150. This, however, gave me 

 no advantage ; I saw nothing more than I had previously 

 observed with the naked eye. San Giovanni has compared 

 the appearance of the structure of the spots to that of felt ; 

 but I could not satisfy myself that this was the case in those 

 examined, although 1 passed through the membrane a very 



