THE SILKWORM ITS INTERNAL STRUCTURE. 41 



really tubes, and they pour the products which are collected 

 within them into the intestine, to be got rid of as part of the 

 excrement. 



Lying partly by the side of, and partly underneath the stomach,, 

 but quite separate from it, are some more twisted tubes (see Fig. 

 13). These are the silk-glands, and there is a pair of them, one 

 on each side. Each consists of three parts ; the central, which 

 will be the first seen, is in the form of a stout, yellowish tube,, 

 bent in three regular folds at the side of the stomach ; this is 

 prolonged behind into a narrower tube which is much twisted 

 about, and in front into an exceedingly fine tube which runs 

 under the gullet straight towards the mouth. Before reaching 

 the mouth, however, it is joined by its fellow of the opposite side, 

 and the two unite to form a single canal which terminates in the 

 spinneret before mentioned. In the dissection there will be no 

 difficulty whatever in following these tubes through the greater 

 part of their course. 



The silk is formed as a gummy secretion, in the lower divisions 

 of the tubes, whence it is conveyed in the shape of exceedingly 

 fine threads along the two straight tubes to the canal formed by 

 their junction. Here the two threads, which of course lie side 

 by side, are bound together into a single one by a gummy secre- 

 tion which is poured into the canal by two little tubes proceeding 

 from t\vo small glands at its sides. It is this gummy secretion 

 that gives the silk the beautiful smoothness and gloss which is 

 one of its chief recommendations as a textile material. Thus it is 

 plain that what appears like a single thread as it issues from the 

 spinneret of the caterpillar, consists in reality of two threads^ 

 lying side by side, but united by a sort of varnish into one. 

 Microscopical examination confirms this conclusion, for if the 

 thread of the cocoon be sufficiently magnified, it is plainly seen 

 to be double throughout. A high degree of magnification, how- 

 ever, is necessary, because the thread, even when double, is of such 

 extreme tenuity that it would require, on an average, no less than 

 two thousand of them laid side by side, to cover a breadth of as 

 much as a single inch. 



Underneath the digestive tube lies the greater part of the 

 nervous system. Though sufficiently large to be able easily to be 

 seen when by itself, it may yet at first be missed, on account of 

 its similarity in colour to the parts over which it lies, and will 

 therefore need careful looking for. It is a most beautiful struc- 

 ture of great delicacy, and its separation from the surrounding 

 parts will require a steady hand and considerable nicety of mani- 

 pulation. To get a rough idea of its form, we may imagine two 



