WHEN we speak of worms, we are referring to a group of animals that is a hotch-potch of living creatures, of very different types, having in common the fact that their bodies are long and thin, that there is little difference between the head and tail ends, and that they crawl about without the aid of legs. Thus, under the headings of worms the uninitiated will include earthworms, shipworms, tape worms, flat worms and an almost infinite variety of unrelated creatures. We have even gone so far as to talk about the blindworm, which is simply a legless lizard. To most of us, however, the earthworm would be regarded as the typical example of a worm, and this will suffice for our present purpose.
The outward form of an earthworm is familiar to all, and calls for no detailed description, but the internal anatomy forms a striking contrast to that of any other animal so far discussed. From the sponges, sea-anemones and jelly-fish, all without intestine or any other of the organs of digestion with which we are familiar in the higher animals, all without a heart or blood-vessels, without brain or nerves or any real sense-organs, we take a big jump in considering the worms, for in them we meet for the first time an anatomy which can be to any degree compared with our own.
The ringed appearance of an earthworm bears a close relation to the rest of the features to be discussed. For example, each ring or segment of the body bears four pairs of short bristles with their bases implanted in the skin and directed outwards, and locomotion is effected by the concerted action of these four rows of bristles which run lengthways along the lower surface of the body. If we pass an earthworm through the thumb and forefinger from the head end backwards these bristles impart a rasping sensation on our skin.
Internally each segment is partitioned from its neighbours, and passing through them all, from the mouth to the hinder end of the body, is a straight tube, the intestine. There is a well-defined central nervous system, with the main nerve running immediately under the intestine, and in each segment this is swollen into a ganglion from which several pairs of nerves are given off to serve the organs in the segment. There is a sort of brain, a simple structure it is true, lying above the intestine
in the front end of the body. In addition the body is served by a system of blood-vessels consisting of three main vessels running longitudinally through the body and connected in front with five pairs of ‘hearts,’ or short pulsating vessels.