Chap. 25 FLATWORMS — VANGUARD OF THE HIGHER ANIMALS 513 



water plants, abundantly on water-chestnuts such as those that were introduced 

 into the United States and have now crowded other plants and animals to 

 extinction in a considerable number of American waterways. 



The success of this parasite's gamble for life has come with the custom of 

 eating water-chestnuts. The outer husk is peeled off and the succulent "nut 

 meat" is eaten raw, an abundant and cheap food in the Chinese summer 

 markets. In China, as many as 1000 larvae of giant flukes have been picked 

 from a single water-chestnut. 



Liver Flukes. A half dozen or more species of liver flukes are frequent para- 

 sites of man mostly in Oriental countries. The Chinese liver fluke, Clonorchis 

 sinensis, is a common parasite of man, cats, and other mammals that eat raw 

 fish. Enclosed in minute capsules, the encysted larvae can live for many 

 months in the muscle of 40 different species of fresh-water fishes thus awaiting 

 a cat or a man to eat them. In an earlier stage, the larvae live in snails. The 

 great numbers of canals and the farm fish ponds in sections of South China and 

 Japan are ideal meeting places for the snails and fishes. The people who eat 

 the fishes give the parasites their final home in the liver. 



Class Cestoda 



The life histories of such parasites as the flukes are mystery stories com- 

 pared with the plain histories of their free-living relatives, the planarians. Para- 

 sitic living has made a still deeper mark on the tapeworm (Cestoda), especially 

 on their appearance. They are hardly recognizable as flatworms and are well 

 named after tape measures. It is believed that any vertebrate may be host to 

 one or another kind of adult tapeworm. 



General Characteristics. The cestodes are internal parasites that are deeply 

 committed to the parasitic habit. Like the trematodes, they have no epidermis; 

 neither have they a mouth or digestive tract, either in immature or mature 

 stages. They have no sensory receptors except free nerve endings that are 

 sensitive to touch. They can move about only feebly, but are amply provided 

 with holdfasts such as hooks and suction cups. In a few species the body is a 

 unit, like those of flukes, but in the great majority it is divided into many units 

 or sections commonly called proglottids from some very highly imagined re- 

 semblance to the shape of the tongue. It is a question whether proglottids 

 might not be more appropriately termed segments since they are repeated as 

 true segments are in the earthworm. The general structure of tapeworms is too 

 degenerate to establish this. 



Adult tapeworms inhabit the intestines of vertebrates entering as larvae, 

 always by way of the mouth. The total length of adults of different species 

 ranges from about that of an ordinary typed hyphen to 40 feet. Like the 

 flukes they require one or more intermediate hosts, vertebrate or inverte- 

 brate, to complete their life history. 



