TRICHINELLA 37 1 



and are carried by the lymph and blood to the muscle, where 

 they remain and grow to a length of i. mm., and become en- 

 closed in a capsule. Unless flesh containing such encapsuled 

 trichina is eaten by another mammal, the development of the 

 worm proceeds no further. When such flesh is eaten the cap- 

 sule is dissolved and the half-grown worm is set free. Thus it 

 gets into the intestine and reaches maturity in the walls of the 

 intestine. This parasite may infest any of the flesh-eating 

 domestic mammals. 



758. There are a large number of intestinal parasites belong- 

 ing to the round worms which infest the intestine of all the do- 

 mestic animals and man. Some of these (Ascaris) reach the 

 size of an earthworm and are very prohfic. The eggs find their 

 way into the digestive tract of a host with the water and food, 

 and their development takes place wholly within the intestinal 

 cavity. These seldom produce an extreme pathological condi- 

 tion in the host. 



759. Many smaller threadworms are parasitic in plants. 



760. The parasitic Arthropods are chiefly found in two or 

 three orders. Among the Entomostraca are the fish flee, chiefly 

 external parasites, and the specially notable case of Sacculina. 

 The adult Sacculina is found attached to the ventral surface of 

 a crab. The portion of it, which is visible externally, is httle 

 more than a large sack containing an elaborately developed 

 reproductive system. The sack is attached to the host by a 

 process of its body, which penetrates the tissues of the host, and 

 then branches and penetrates in all directions, hke the root 

 system of a plant. By this organ it absorbs nourishment from 

 the host. It has no digestive system. The young of this strange 

 organism are free-swimming nauplii with eyes and appendages 

 of the typical naupHus. But when the larvae attach them- 

 selves to a host, the appendages and eyes undergo degenera- 

 tion until there remains only the organism as described. 



761. Among Insects, the parasites are found chiefly among 



