UNIVALVES. 
49 
portion descends obliquely behind the mouth, 
and its edges are united to form a tube (c), and 
enclosed in a membranous sheath, which opens 
gradually as the part is brought forward to 
replace the worn portion. 
The most simple plan to prepare these as 
microscopic objects is to boil the head of the 
mollusk in a solution of potash in a test-tube, 
by which all the parts, with the exception of the 
tongue and jaw, are dissolved : care must be taken 
to thoroughly wash the tongue before mounting. 
The most instructive method is doubtlessly that 
of dissection ; but certainly, when we have some 
of our minute snails to deal with, that of mace¬ 
ration will be a great saving of time and patience. 
The head should be pinned down in a gutta¬ 
percha trough containing water enough to cover 
the part ; the floor of the mouth may be laid 
open by passing the lower point of a pair of 
scissors into the mouth, and cutting upwards; 
now pin back the severed portions, and by the 
aid of a lancet or needle, work out the lingual 
apparatus. The ribbon should be cleaned by 
washing with a camehs hair brush, or by soaking 
in potash-water: if the latter, wash the tongue 
well before mounting. The preparation may be 
mounted in glycerine, or if intended as an object 
for the polariscope, it should be mounted in 
Canada balsam. 
