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, 

 i.e., about fourteen square feet. The beauty of these is, 

 of course, not perfectly shown until they are injected, which 

 will be noticed elsewhere. 



TONGUES, OR PALATES, OF MOLLUSCS. 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. On 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 : The 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, 



