THE SILKWORM ITS INTERNAL STRUCTURE. 39 



operator will, especially at his first few 

 attempts, have cut through and irretriev- 

 ably damaged a long, fine tube that 

 lies just under the skin all along the 

 middle of the back, and is therefore 

 called the " dorsal vessel." This is the 

 creature's heart, but as there is some 

 difficulty in recognising it when dis- 

 secting from above, we will neglect it 

 for the present, and suppose that we 

 have cut through and spoilt it. 



We must now, with the point of the 

 scalpel or with the needles aided by the 

 forceps, gently remove the "fat body" 

 as much as possible from the underlying 

 organs. As we do this we shall find 

 that we lay bare a long tube which runs 

 straight along the body, from one end 

 to the other, and in its diameter is more 

 than half as big as the body itself (Fig. 

 13). This is the digestive tube, into 

 which the food is received, in which it 

 is digested, and from which the nutri- 

 ment it contains soaks through to the 

 body at large. It appears of a dark 

 colour ; but this is a deceptive appear- 

 ance ; it is really a pale tube, but, as it 

 is semi-transparent, the dark mass of the 

 food it contains shows through its walls 

 and produces the dark tint. We shall 

 presently wash out the contained food, 

 and shall then be able to see the real 

 colour of this great and important organ. 

 Meanwhile, we notice that the tube 

 seems anchored down on each side by 

 a series of extremely fine threads, re- 

 minding one of way in which the tiny 

 inhabitants of Liliput pegged Gulliver 

 down to the ground by the hairs of his 

 head, while he was lying asleep on his 

 first arrival in their country. These 

 fine threads, when they reach the diges- 

 tive tube itself, begin to branch and 

 spread out like fine roots over the sides. 





; ,' 



L, I 



