42 SILKWORMS. 



fine threads of white cotton of equal length, placed side by side, 

 and then, at regular intervals down their whole length, a series of 

 knots tied across the two combined. Thus we should have a 

 kind of chain of knots, or lumps, connected by two parallel 

 threads. This is something of the sort of structure we have to 

 look for. It is to be found all along the middle line of the body 

 (Fig. 14 />), immediately beneath the digestive tube, but quite 

 separate from that throughout the greater part of its length. The 

 little swellings of nervous matter, which correspond to the knots 

 on our threads, are called ganglia, and therefore the central mass 

 of the nervous system is usually described as a " double chain of 

 nervous ganglia." 



By the removal of the digestive tube in the manner indicated 

 above, we shall be pretty sure to have torn and spoilt the begin- 

 ning of this chain, which is closely connected with that tube. If, 

 however, before removing it, we had examined carefully that part 

 of it immediately behind the mouth, we should have noticed on 

 its upper surface two small, rounded, whitish knobs, very near 

 together, and connected with one another ; and a closer examina- 

 tion, aided by the hand-lens, would have shown two delicate, 

 whitish threads proceeding from them, and passing, one from 

 each knob, down the sides of the gullet, until they united with 

 another pair of knobs beneath. Thus a nervous ring is formed 

 round the gullet, with a large pair of ganglia above, and a smaller 

 pair beneath. The larger pair above is sometimes called a 

 " brain," and though this may not be a very suitable name for it, 

 yet it serves to indicate that from this pair proceed nerves to such 

 organs of sense as the creature possesses, and that thus it is to 

 the caterpillar, in some measure, what the real brain of a quad- 

 ruped is to it. It is this upper pair of ganglia which one can 

 scarcely avoid tearing off when the digestive tube is removed, 

 unless special pains be taken to preserve them. To do this, we 

 must carefully cut through the gullet just behind them, but far 

 enough away to avoid injuring the nervous threads which join the 

 two pairs of ganglia, and then the nervous ring or collar can be 

 slipped off the remaining portion of the gullet without damage to 

 itself, and without being separated from the rest of the chain. 



This being done, we notice that though the two threads joining 

 the first two pairs of ganglia are wide apart, to allow of the gullet 

 passing between them, those of the rest of the chain are much 

 closer together, and parallel to one another. Each little swelling 

 on the chain consists of a pair of ganglia, placed close together 

 .and united with one another by threads, and from each pair there 

 pass a few fine threads to different parts of the segment in which 



