OF MICROSCOPIC OBJECTS. Ill 



dissecting-trough before mentioned, and the head and forepart cut 

 open, spread out, and firmly pinned down. With the aid of fine 

 scissors or knife, the tongue must be then detached from its fasten- 

 ings, and placed in water for a day or two, when all foreign matter 

 may with a little care be removed. In what way it should be 

 mounted will depend on the purpose for which it is intended. If 

 for examination as an ordinary object, it may be laid upon the 

 slide and allowed to dry, which arrangement will show the teeth 

 very well. If we wish to see it as it is naturally, it must be 

 mounted in a cell with Goadby's fluid ; but if it is wanted as a 

 polarizing object, it must be floated upon a slide, allowed to dry 

 thoroughly, and then Canada balsam added in the usual manner. 



In the stomach, also, of some of these molluscs teeth are found, 

 which are very interesting objects to examine, and must be dis- 

 sected out in the same manner as the " tongues." 



Since writing the above, Dr. Alcock (whose very beautiful 

 specimens prove him to be a great authority in this branch) has 

 published some of his experience in the second volume of the 

 third series of " Memoirs of the Literary and Philosophical Soci- 

 ety of Manchester." By his permission I make the following 

 extract : 



" This closes my present communication on the tongues of mol- 

 lusca ; but as some members may possibly feel inclined to enter 

 upon the inquiry themselves, I think it will not be amiss to add a 

 few remarks on the manner in which they are to be obtained. 



" First, as to the kinds best worth the trouble of preparation. 

 Whelks, Limpets, and Trochuscs should be taken first. Land 

 and fresh-water snails can scarcely be recommended, except as a 

 special study, their tongues being rather more difficult to find, 

 and the teeth so small that they require a high power to show 

 them properly. It would appear, from Spallanzani's description 

 of the anatomy of the head of the snail, that even he did not make 

 out this part, although, in his curious observations on the repro- 

 duction of lost parts, he must have carefully dissected more snails 

 than any other man. 



"As to preserving the animals till wanted, they should simply 

 be dropped alive into glycerine or alcohol. Glycerine is perhaps 

 best where only the tongues are wanted ; but it leaves the ani- 



