DESCRIPTION OF PLATES. 203 
TreetH oF Motuusca. Puate XX. Fic. 1. 
By careful dissection, the student will be able to 
obtain from the Mollusca a variety of interesting 
objects for study under the microscope. Most 
attractive and curious among these are the teeth 
upon the lingual membrane of these creatures. 
As a class they are vegetable feeders, and these 
sharp teeth, arranged in rows upon the surface of 
the tongue, enable them to rasp away the surface 
of the succulent leaves upon which they feed. In 
the species from which the specimen was obtained 
which has furnished our photo-micrograph, it is 
estimated that the number of teeth exceeds twenty 
thousand. 
TracHes& oF Insects. PuLate XX. Fig. 2. 
The trachez of Insects and of their larve are 
branching cylindrical tubes, which take their 
origin from the spiracles, which are apertures sit- 
uated upon the lateral and upper portions of the 
abdomen, and at the posterior, lateral, and upper 
parts of the thorax. These branching tubes are 
distributed to all parts of the body, and serve to 
convey air to the fluids and tissues. They consist 
of two coats, between which lies a spiral fibre which 
is well seen in the photo-micrograph. 
Tn conclusion, the Author ventures to hope that 
the elementary facts in biology and in histology 
