Tracheæ of Insects, &c.—The nature of these was described in [Chapter III.], but the method of procuring them was not explained, as this clearly belongs to “dissection.” The larger tubes are readily separated by placing the insect in water, and fixing as firmly as possible, when the body must be opened and the viscera removed. The tracheæ may then be cleaned by the aid of a camel-hair pencil, and floated upon a glass, where they must first be allowed to dry, and then be mounted in balsam. Mr. Quekett gives the following method of removing the tracheæ from the larva of an insect:—“Make a small opening in its body, and then place it in strong acetic acid. This will soften or decompose all the viscera, and the tracheæ may then be well washed with the 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 tracheæ 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 tracheæ 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 larvæ 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 dissecting-trough before mentioned, and the head and forepart cut open, spread out, and firmly pinned down. With the aid of fine scissors or knife, the tongue must be then detached from its fastenings, and placed in water for a day or two, when all foreign matter may with a little care be removed. In what way it should be mounted will depend on the purpose for which it is intended. If for examination as an ordinary object, it may be laid upon the slide and allowed to dry, which arrangement will show the teeth very well. If we wish to see it as it is naturally, it must be mounted in a cell with Goadby’s fluid; but if it is wanted as a polarizing object, it must be floated upon a slide, allowed to dry thoroughly, and then Canada balsam added in the usual manner.
In the stomach, also, of some of these molluscs teeth are found, which are very interesting objects to examine, and must be dissected out in the same manner as the “tongues.”
Since writing the above. Dr. Alcock (whose very beautiful specimens prove him to be a great authority in this branch) has published some of his experience in the second volume of the third series of “Memoirs of the Literary and Philosophical Society of Manchester.” By his permission I make the following extract:—
“This closes my present communication on the tongues of mollusca; but as some members may possibly feel inclined to enter upon the inquiry themselves, I think it will not be amiss to add a few remarks on the manner in which they are to be obtained.
“First, as to the kinds best worth the trouble of preparation. Whelks, Limpets, and Trochuses should be taken first. Land and fresh-water snails can scarcely be recommended, except as a special study,—their tongues being rather more difficult to find, and the teeth so small that they require a high power to show them properly. It would appear, from Spallanzani’s description of the anatomy of the head of the snail, that even he did not make out this part, although, in his curious observations on the reproduction of lost parts, he must have carefully dissected more snails than any other man.
“As to preserving the animals till wanted, they should simply be dropped alive into glycerine or alcohol. Glycerine is perhaps best where only the tongues are wanted; but it leaves the animals very soft; and as it does not harden their mucus at all, they are very slippery and difficult to work upon when so preserved.
“Then as to the apparatus required for dissection. In the first place, all the work is to be done under water, and a common saucer is generally the most convenient vessel to use. No kind of fastening down or pinning out of the animal is needed; and, in fact, it is much better to have it quite free, that you may turn it about any way you wish. The necessary instruments are a needle-point, a pair of fine-pointed scissors, and small forceps; the forceps should have their points slightly turned in towards each other.
“A word or two on the lingual apparatus generally, and on its special characters in a few different animals, will conclude what I have to say.
“The mode of using the tongue can be easily seen in any of the common water-snails, when they are crawling on the glass sides of an aquarium; it may then be observed that from between the fleshy lips a thick mass is protruded, with a motion forwards and upwards, and afterwards withdrawn, these movements being almost continually repeated. The action has the appearance of licking; but when the light falls suitably on the protruded structure, it is seen to be armed with a number of bright points, which are the lingual teeth, so arranged as to give the organ the character and action of a rasp.