162 HEALTH AND DISEASE 



Helminths (Worms). — Between the highly organized parasites which 

 are descriljcd a.s 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 tlat-worms (flat helminths). There 

 are two varieties of them, which are known as tape- worms and fluke- 

 worms; the latter being entirely unlike the tyjjical 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 

 oroans 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 



