400 



ANIMAL BIOLOGY 



vesicles, or hydatids, usually with fatal results. Such larvae in the 

 brains of sheep were a stumbling-block for the early exponents of 



Egg membrane 

 Hooks 

 Egg 



A B C 



Fig. 253. — Stages in the development of a Tapeworm. A, egg with embryo; 

 B, bladder worm (cysticercus) before head (scolex) is protruded; C, same 

 after protrusion. (From Hegner.) 



biogenesis, since, with the life history unknown, they could not 

 account for the larvae except on the theory of spontaneous gen- 

 eration. 



Nematodes. Passing now to the Nematoda, or Roundworms, 

 we come to a group which, from the standpoint of medical zoology, 

 is of as much importance as the Flatworms. Free-living forms are 

 found literally everywhere in water, soil, and air, and blown about 

 by the wind. Most of these are harmless, but some are of great 

 economic interest because of their destructive action on the roots 

 and other parts of plants. Among the species parasitic in Man and 

 the higher animals, Trichinella and the Hookworm will serve as 

 examples. (Fig. 44.) 



Trichinella is the cause of a serious disease in Man, Pigs, and 

 Rats known as trichinosis. Man becomes parasitized by eating 

 infected pork, insufficiently cooked, and pigs contract the disease 

 by eating offal or infected rats. The larvae from the meat quickly 

 mature in the human intestine, and each female worm produces 

 nearly ten thousand larvae which bore through the intestinal wall, 

 migrate throughout the voluntary muscles, and encyst there to 

 await a possible getaway from the body at death. Since thou- 

 sands of resistant cysts may occur in a single gram of muscle, the 

 riddling of the tissues is not only very serious, but incurable. 

 (Figs. 250, 254.) 



