Between the highly organized parasites which are described as worms and the elementary forms which have just been referred to there do not appear to be any connecting links; at least none have been discovered. The word worm is popularly believed to indicate creatures of which the earth-worm is an example, but the helminthologist includes in the term animals which differ considerably from each other in form, sufficiently, at least, to lead to their division into two sub-classes, the characters of which are very easily distinguished.

In the first subdivision - round-worms or helminths - all the members have the characteristic forms of the common earth-worm, that is to say, they are round and elongated; but they differ in length from an eighth of an inch to several feet, in other words, from very large to extremely minute worms which can only be seen by the aid of the microscope.

Round-worms are found in various parts of the body of their host - in the skin, the eye, stomach, intestines, in the kidneys, and occasionally in the heart and blood-vessels.

The second subdivision includes all flat-worms (flat helminths). There are two varieties of them, which are known as tape-worms and fluke-worms; the latter being entirely unlike the typical worm, as it resembles a very minute sole, and when it is fully grown it is little more than an inch in length.

Both round-worms and flat-worms produce eggs in which embryos are developed, but it is characteristic of all the division that the young worms do not become mature in the organism of the animal which they infest; in some cases the young worm is hatched out in the intestines or other-organs of the host, but before it is fully developed it appears to be necessary that it should be expelled and find an intermediate host outside the animal in which it has lived. In the case of the round-worm this phase in their life-history is still obscure; the intermediate host has not been discovered. In the flat-worms, on the other hand, the changes have been followed from the egg through the body of the intermediate host back to the class of animal originally infested. In the fluke, for example, the embryo bores its way into a small snail, in which it goes through certain changes of form. In the case of the tape-worm the embryo in the body of an intermediate host becomes a hydatid, which is really a small bladder in which the tape-worm head is developed. The transference of the hvdatid to the body of another host is followed by the development of the tape-worm from the head or heads. The only mystery which exists in reference to this parasite relates to the identification of the host, which in a considerable number of tape-worms is not known, although, judging from analogy, there can be no doubt of its existence. To make this mode of development clear a simple illustration will be sufficient. A certain hydatid infesting the brain of the sheep, which causes the disease known as gid, will, if given to a dog, be developed into a tape-worm possessing certain well-defined characters which mark its origin. The mature segments of this variety of tape-worm, when given to sheep or taken up by them when feeding on pastures, will produce the hydatid in the brain.