206 



ASSOCIATION OF ORGANISMS THE WEB OF LIFE 



gigas, fig. 1 1 54), which, when adult, lives in the intestine of the 

 pig, maintaining a firm hold by means of a formidable hollow 

 "proboscis", thickly studded with hooks. The eggs pass out of 

 the body of the pig, and some of them are swallowed by beetle- 

 grubs, within which the development is carried on to a certain 

 stage. If an infested grub happens to be eaten by a pig the life- 

 history of the parasite is completed. 



ANIMALCULES (PROTOZOA) AS. PARASITES 



The large group of Gregarines (Sporozoa) includes typical 

 internal parasites, in which the body is surrounded by a firm 

 elastic membrane, through which the body-fluids or digested food 



of the host are easily 

 absorbed. A well-known 

 form ( Clepsidrina blat- 

 tarum, fig. 1155) infests 

 the intestines of the 

 cockroach. The young 

 parasite is worm - like 

 in shape, and its body 

 is divided into three 

 regions, one of which 

 bears hooks, and serves 

 as a means of adhesion 

 to the host. Later on 

 this part is thrown off, 

 and the gregarine lies 

 freely in the intestine, 



where it absorbs food, and increases considerably in size. The 

 life-history is somewhat peculiar. Two parasites adhere together, 

 and become enclosed in a firm case or cyst, which is passed out 

 of the body of the host. If the surroundings are sufficiently damp 

 the outer part of the cyst swells up into a gelatinous layer, and 

 complex changes go on in the interior. The bodies of the two 

 contained parasites break up into a large number of minute spores, 

 with firm coats, scattered through a kind of net-work. A number 

 of tubes are also formed which turn inside out, and since they 

 convey the spores out of the cyst are termed sporoducts. If 

 these spores are swallowed by a young cockroach their firm coats 



g- 



Fig. 1155. Cockroach Gregarine (Clepsidrina blattarunt], enlarged 

 to various scales. A, Young gregarines attached to cells of intestine; 

 B and c, later stages lying free within intestine; D, spore-cyst in section. 

 i, 2, 3, Three body-regions, of which i is thrown off by adult; ., 

 nucleus; g., gelatinous covering of cyst; sp., mass of spores in middle 

 of cyst ; sp. t. , spore-tubes ; w. , firm wall of cyst. 



