40 SILKWORMS. 



These are some of the breathing tubes, and as we shall have 

 occasion to notice them more particularly presently, we will) 

 now cut them through and so release the digestive tract from 

 its moorings. If now we cut through the latter just behind 

 its commencement, we shall be able, by gently waving it about 

 in the water, to wash out a good deal of its contents, and to turn 

 it on one side and so examine it more satisfactorily. It is then 

 seen to be, throughout the upper two-thirds of its course, a very 

 thin-walled tube, while the hinder third has walls a good deal 

 thicker than the rest. 



The part immediately behind the mouth is called, naming it 

 it after the corresponding structure in human anatomy, the ceso- 

 phagus, or gullet. The food does not remain in this, but it simply 

 serves as a passage into the stomach. Succeeding it comes the 

 stomach itself, the true digestive cavity, where the coarsely masti- 

 cated food is received to be submitted to chemical change. This 

 occupies by far the greater part of the tube, and indeed fills so- 

 large an amount of the space enclosed by the body walls, that the 

 caterpillar might, not inaptly, be described as one great stomach. 

 The enormous extent of this portion of the animal's organization 

 is, of course, the key to its extraordinary appetite. The stomach 

 is succeeded by the intestine, the diameter of which is at first less 

 than that of the preceding part, though just before the termination 

 of the tube it enlarges again. In this the undigested remains of 

 the food are received and stored up, and by the pressure of its 

 muscular walls compacted into little cylindrical pellets with 

 grooves down the sides, which are periodically expelled from the 

 end of the intestine as excrement. The walls of part of the 

 intestine are raised inside into a number of prominent muscular 

 ridges which project into its cavity, and can easily be seen by 

 slitting open with the scissors this division of the digestive tube. 

 These ridges are the causes of the grooves down the sides of the 

 excrementitious pellets above alluded to. The wide terminal 

 portion of the intestine is called the rectum. 



At the junction of the intestine with the stomach there are on 

 each side six very long and exceedingly fine tubes projecting from 

 the outside, some parts of which are laid neatly in longitudinal 

 folds along the hinder part of the walls of the stomach (see Fig. 13), 

 and others packed closely together like so many tangled threads 

 just opposite the point of their insertion. They are the so-called 

 Malpighian tubes, thus named after a celebrated Italian anatomist. 

 They are considered to belong to the excretory system, and 

 indeed, to have a similar function to the kidneys of the higher 

 animals. Though they appear simply like threads, they are 



