PARASITES AND THEIR DEVELOPMENT. 



187 



eggs as do not reach water, will not undergo development, 

 and hence a first check to the increase of the flukes exists 

 in the fact that many eggs must perish from the absence of 

 appropriate surroundings. Sooner or later, the young fluke 

 loses its power of swimming, and becomes of oval shape; 

 crawling inelegantly, by contractions of its body, over the 

 muddy bottom of its pool or river. Thereafter it appears to 

 seek an entrance to the body of some co-tenant of its pool, 

 such a creature being usually found in the shape of a water- 

 snail. Buried within the tissues of this first " host," the 

 young fluke becomes transformed into a sac 

 or bag, within which other young may arise 

 by a veritable process of budding. This 

 rising generation appears in the form of small 

 bodies, each provided with a vibratile tail. 

 From the body of the snail, these " secondary 

 young " soon make their escape ; and whilst 

 existing in the water, are readily conveyed 

 into the stomach of the sheep in the act of 

 drinking. Thence these young flukes pene- 

 trate to the liver of the animal, and become 

 transferred into the mature and flattened 

 adult. 



The unexplained necessity for such a 

 complicated series of changes in develop- 

 ment, and for the varied circumstances which 

 mark the career of the young fluke, present 

 us with conditions which operate powerfully 

 against the undue increase of the race. An 

 exactly analogous series of changes is to be 

 perceived in the development of many other 

 parasites, and amongst others in that of the 

 various groups of tapeworms (Figs. 18 and 

 19), which reside within the digestive system 

 of man and other quadrupeds, and which are 

 in reality "compound" animals, each joint being a semi- 

 independent unit of the compound being. But for the 



FIG. 1 8. Tapeworm 

 ( Taenza echinococ- 

 cus) of dog, in its 

 mature condition: 

 a, head, with suck- 

 ers ; 6, a full-grown 

 joint; c, aperture 

 by which the .eggs 

 escape. 



