WORMS. 



193 



Vjo. 172. — Cys- 

 togenous cells. 



distiuguish portions of the digestive tract and numerous cells, three of which with 

 tiieir granular contents are represented very much magnified in Fig. 172. These cells 

 have a glandular character and serve to pour out a mucous secretion, the 

 functi6n of which will be immediately described. There may be as 

 many as twenty-three spores in various stages of development in one 

 redia; out of these spores there Vifill be one, two, or three ceroarise 

 approaching complete development. 



As soon as the cercaria has i-eached the limit of its development 

 within the redia, it escapes from the parent by the birth opening. When free, the 

 cercaria. Fig. 171, is very active, and constantly changes its form. By the aid of its 

 suckers, the tadpole-shaped larva crawls or wriggles its way out of its host. When the 

 infested cells are kept in an aquarium, the cercariae may be found occasionally swim- 

 ming about in the water ; but not long, for on coming in contact with the side of the 

 aquarium or with water-plants, it proceeds to encyst itself. The process can be readily 

 observed under the microscoj:)e ; for, on a glass slide, the cercaria soon comes to rest. 

 It assumes a rounded, form, whilst a mucous substance is poured forth over the body 

 together with the granules of the cystogenous cells, which we mentioned above. The 

 tail is shaken off either before or during encystation, which is com- 

 pleted in a few minutes. These cysts are the means of infecting the 

 final vertebrate host of the parasites, the infection being rendered 

 possible by the habits of the intermediate host, -Limnceiis truncatulus, 

 which might well be termed amphibious, so strongly is its habit of 

 wandering on land developed. Indeed they can remain on land for 

 long periods, and resist even prolonged droughts : hence when in the 

 water, the snails become infested, and when on land, leave the cer- 

 cai-iae that crawl out of their first host scattered over the fields, where 

 they encyst on the grass and are eaten by the sheep and other animals. 

 In the stomach of the unlucky sheep the cyst is dissolved, leaving 

 the worm free. The worm then makes its way into the liver, and 

 probably in about six weeks begins to produce eggs, growing itself 

 meanwhile. During its growth its external form changes, the simple 

 forked intestine develops numerous blind secondary 

 branches, the posterior sucker is greatly enlarged and the sexual 

 oi'gans are matured. Thereafter the wondrous cycle of metamor- 

 phoses and emigration recommences with the new eggs. There are, 

 perhaps, no instances more striking of the adaptation of animal species 

 to particular conditions of existence than we encounter in the life 

 histories of trematode worms, one of which we have narrated. 



The adult worm, Distomum /lepaticum, attains a length of three- 

 quarters of an inch ; it is broad and flat and at its anterior end has a 

 small projecting lobe, which bears two ventral suckers, one in front 

 at the very extremity surrounds the mouth, the second one consider- 

 ably larger lies an eighth of an inch or more fui-ther back. It is found 

 in most ruminants and in many other animals including man; it is 

 commonly encountered in the biliary ducts, but is sometimes found 

 in the intestine or venous system. Geographically it is very widely 

 distributed not only throughout Europe and America, but also in Egypt, India, and even 

 Australia and Van Diemen's Land. Although discovered by Gabucinus as long ago as 



VOL. I. — 13 



Fig. 173. — Young 

 Distomum ; b, 

 pharynx; c, sto- 

 mach; o, mouth; 

 s, suckers. 



Pig. 174. — Distomuiit 

 hcpaticum, liver 

 fluke. 



