THE FLATWORMS 191 



as an adult in the dog, may form large vesicles in man, known 

 to physicians as hydatides (Fig. 112, C), which may break with 

 serious or even fatal results. The organism which causes " gid " 

 or " staggers " in sheep (Fig. 112, B) is the larva of the dog 

 tapeworm, Tania ccenurus. It becomes lodged in the brain or 

 spinal cord. Goats, cattle, and deer are also attacked by the 

 same species. 



Characteristics and Classification. — The flatworms belong 

 to the phylum Platyhelminthes. They are unsegmented like the 

 roundworms, but can be distinguished from the latter by the 

 flattened condition of the body. The alimentary canal has only 

 one external opening, the mouth. Food substances enter the 

 mouth, and undigested particles are also cast out of this opening. 

 In the tapeworm there is no mouth at all, the food being absorbed 

 by the general body wall. All flatworms are hermaphroditic, 

 since each individual is provided with both male and female 

 reproductive organs. The parasitic habit of many flatworms 

 has led to complicated life histories such as that of the liver fluke. 



The three classes of the phylum are as follows: — 



Class 1. Turbellaria. — Free-living animals like Planaria. 



Class 2. Trematoda. — Parasitic animals like the liver 

 fluke. 



Class 3. Cestoda. — Parasitic animals like the tapeworm. 



REFERENCES 



Cambridge Natural History, Vol. II. — The Macmillan Co., N. Y. City. 

 Bulletins published by the Bureau of Animal Industry, U. S. Department 

 of Agriculture. 



