Structure.--A few remarks must be made on this subject.The body of a large worm consists of from 100 to 200 almost cylindrical rings or segments, each furnished with minute bristles.The muscular system is well developed.Worms can crawl backwards as well as forwards, and by the aid of their affixed tails can retreat with extraordinary rapidity into their burrows.The mouth is situated at the anterior end of the body, and is provided with a little projection (lobe or lip, as it has been variously called) which is used for prehension.Internally, behind the mouth, there is a strong pharynx, shown in the accompanying diagram (Fig.1) which is pushed forwards when the animal eats, and this part corresponds, according to Perrier, with the protrudable trunk or proboscis of other annelids.The pharynx leads into the oesophagus, on each side of which in the lower part there are three pairs of large glands, which secrete a surprising amount of carbonate of lime.These calciferous glands are highly remarkable, for nothing like them is known in any other animal.Their use will be discussed when we treat of the digestive process.In most of the species, the oesophagus is enlarged into a crop in front of the gizzard.This latter organ is lined with a smooth thick chitinous membrane, and is surrounded by weak longitudinal, but powerful transverse muscles.Perrier saw these muscles in energetic action; and, as he remarks, the trituration of the food must be chiefly effected by this organ, for worms possess no jaws or teeth of any kind.Grains of sand and small stones, from the 1/20 to a little more than the 1/10 inch in diameter, may generally be found in their gizzards and intestines.As it is certain that worms swallow many little stones, independently of those swallowed while excavating their burrows, it is probable that they serve, like mill-stones, to triturate their food.The gizzard opens into the intestine, which runs in a straight course to the vent at the posterior end of the body.The intestine presents a remarkable structure, the typhlosolis, or, as the old anatomists called it, an intestine within an intestine; and Claparede has shownthat this consists of a deep longitudinal involution of the walls of the intestine, by which means an extensive absorbent surface is gained.
The circulatory system is well developed.Worms breathe by their skin, as they do not possess any special respiratory organs.The two sexes are united in the same individual, but two individuals pair together.The nervous system is fairly well developed; and the two almost confluent cerebral ganglia are situated very near to the anterior end of the body.
Senses.--Worms are destitute of eyes, and at first I thought that they were quite insensible to light; for those kept in confinement were repeatedly observed by the aid of a candle, and others out of doors by the aid of a lantern, yet they were rarely alarmed, although extremely timid animals.Other persons have found no difficulty in observing worms at night by the same means.
Hoffmeister, however, states that worms, with the exception of a few individuals, are extremely sensitive to light; but he admits that in most cases a certain time is requisite for its action.These statements led me to watch on many successive nights worms kept in pots, which were protected from currents of air by means of glass plates.The pots were approached very gently, in order that no vibration of the floor should be caused.When under these circumstances worms were illuminated by a bull's-eye lantern having slides of dark red and blue glass, which intercepted so much light that they could be seen only with some difficulty, they were not at all affected by this amount of light, however long they were exposed to it.The light, as far as I could judge, was brighter than that from the full moon.Its colour apparently made no difference in the result.When they were illuminated by a candle, or even by a bright paraffin lamp, they were not usually affected at first.Nor were they when the light was alternately admitted and shut off.Sometimes, however, they behaved very differently, for as soon as the light fell on them, they withdrew into their burrows with almost instantaneous rapidity.This occurred perhaps once out of a dozen times.When they did not withdraw instantly, they often raised the anterior tapering ends of their bodies from the ground, as if their attention was aroused or as if surprise was felt; or they moved their bodies from side to side as if feeling for some object.They appeared distressed bythe light; but I doubt whether this was really the case, for on two occasions after withdrawing slowly, they remained for a long time with their anterior extremities protruding a little from the mouths of their burrows, in which position they were ready for instant and complete withdrawal.
When the light from a candle was concentrated by means of a large lens on the anterior extremity, they generally withdrew instantly; but this concentrated light failed to act perhaps once out of half a dozen trials.The light was on one occasion concentrated on a worm lying beneath water in a saucer, and it instantly withdrew into its burrow.In all cases the duration of the light, unless extremely feeble, made a great difference in the result; for worms left exposed before a paraffin lamp or a candle invariably retreated into their burrows within from five to fifteen minutes; and if in the evening the pots were illuminated before the worms had come out of their burrows, they failed to appear.