OF MICROSCOPIC OBJECTS. 169 
It is not a very difficult matter to open the head and remove 
the gills, which are very beautiful. Under the outer covers 
lie a quantity of thin plates or leaves (as of a book) which 
in different fishes are of various shapes, but are made like 
net-work by the numerous veins and arteries which convey 
the blood to be acted upon by the air and gases in the 
water, as is done in the lungs of a man. ‘These plates are 
of such numbers that in a good-sized salmon the surface 
exposed has been estimated at two thousand square inches, 
¢.e., about fourteen square feet. The beauty of these is, 
of course, not perfectly shown until they are injected, which 
will be noticed elsewhere. 
Toncuzs, OR Patates, oF Motztuscs.—Of the nature of 
these, Dr. Carpenter gives the following description :—‘ The 
organ which is commonly known under this designation is 
one of a very singular nature; and we should be altogether 
wrong in conceiving of it as having any likeness to that on 
which our ordinary ideas of such an organ are founded. 
For, instead of being a projecting body, lying in the cavity 
of the’ mouth, it is a tube that passes backwards and down- 
wards beneath the mouth, its higher end being closed, 
whilst in front it opens obliquely upon the floor of the 
mouth, being, as it were, slit up and spread out so as to 
form a nearly flat surface. Ox the interior of the tube, as 
well as on the flat expansion of it, we find numerous trans- 
verse rows of minute teeth, which are set upon flattened 
plates ; each principal tooth sometimes having a basal plate 
of its own, whilst in other instances one plate carries 
several teeth.” These palates, or tongues, differ much 
amongst the Gasteropods in form and size, some of them 
being comparatively of an immense length. Many are 
amongst the most beautiful objects when examined with 
polarized light. They must, however, be procured by dis- 
section, which is usually performed as follows :—Tbe animal 
is placed on the cork in the dissecting-trough before men- 
tioned, and the head and forepart cut open, spread out, and 
firmly pinned down. With the aid of fine scissors or knife, 
