110 PREPARATION AND MOUNTING 



syringe, and removed from the body with the greatest facility, by 

 cutting away the connections of the main tubes with the spiracles 

 by means of fine-pointed scissors. In order to get them upon the 

 slide, it must be put into the fluid, and the tracheae floated upon 

 it ; after which they may be laid out in their proper position, then 

 dried and mounted in balsam." If we wish them to bear their 

 natural appearance, they must be mounted in a cell with Goadby's 

 fluid; but the structure is sometimes well shown in specimens 

 mounted dry. As before mentioned, these tracheae terminate on 

 the outside in openings termed spiracles, which are round, oblong, 

 and of various shapes. Over these are generally a quantity of 

 minute hairs, forming a guard against the entrance of dust, &c. 

 The forms of these are seldom alike in two different kinds of insects, 

 so that there is here a wide field for the student. The dissection, 

 moreover, is very easy, as they may be cut from the body with a 

 sharp knife or scissors, and mounted in balsam or fluid. Many of 

 the larvae afford good specimens, as do also some of the common 

 Coleopterous insects. 



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 downwards 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 transverse 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 dissection, which is usually 

 performed as follows : The animal is placed on the cork in the 



