264 PRACTICAL LESSONS IN SCIENCE. 



ternal parasites. The mouth is in the middle of a small sucker, 

 and frequently there are small hooks of chitin which may assist 

 the worm in attaching itself to desirable objects, and sometimes 

 there are several suckers. The best known is the liver-fluke, 

 which infests the ducts of the liver in sheep. The fertilized ova 

 produce ciliated embryos, which wander about till they die or 

 find a host, usually a snail, in which they are transformed into 

 sporocysts, which produce redise, that give rise to a generation 

 of tailed cercaria. In this form they leave the snail and encyst 

 on some foreign object, as a blade of grass ; in this state they 

 are swallowed by grazing sheep, and soon form mature flukes in 

 their livers. 



The Cestoda are the tape worms. They have flattened bodies 

 without mouth or alimentary canal. They infest several different 

 animals, but Tssnia solium, which sometimes inhabits the intes- 

 tines of man, is perhaps the most interesting one to study. It 

 consists of a large number of flattened joints, each of which is a 

 complete hermaphrodite organism. At one extremity the organ- 

 ism is fixed to the mucous membrane of the intestine by means 

 of a crown of hooks and suckers. This part is called the head ; 

 the other parts are produced from the head by a process of bud- 

 ding. Each joint is well supplied with eggs. When mature the 

 joint breaks off and is expelled from the body. The eggs are 

 swallowed by the pig; the coat of the egg of the tape-worm is 

 dissolved and the embryo is set free. It soon passes through the 

 walls of the stomach to some of the internal organs of the pig, 

 where it forms a cyst, giving rise to the disease known as measles. 

 Meat containing these cysts is eaten, the worm is liberated from 

 the cyst and fixes itself to the intestine and develops into an 

 adult tape worm. The tape-worm of the cat is the mature cystic 

 worm of the mouse. The tape worm is nourished by absorbing 

 the nutrient fluids elaborated by its host. 



The Nematoda are the round worms and thread worms which 

 are parasites in the lower part of the human intestine, and they 

 include about two hundred species that are not parasitic. The 

 so-called vinegar eel is a familiar member of this group. The 



