192 



LOWER INVERTEBRATES. 



Fig. 1G9.— Cyst 

 of Blstoma. 



appears to have some means of instinctively recognizing the trunculatus, for it does 

 not attack other species of snails. It cannot live much more than twelve hours in 

 water, and it usually gets into a snail within eight hours. In its host the embryo 

 changes into a new form, the nurse or sporocyst, within which aiise the 

 germs or spores producing new individuals. The outer ciliated cells 

 swell up, and are finally east off. The embryo then becomes a little 

 bag or cyst, at one end of which the ])igmented eye-spots of the. embryo 

 can still be recognized. The cyst or nurse grows and elongates. During 

 warm summer weather it may reach its full size within a fortnight, but 

 in autumn twice that time may be necessary. These cysts sometimes, 

 but rarely, multiply by transverse division, but in other species this 

 phenomenon is more frequent. 



The next larval forms, the redise, are developed within the sporocyst. The first 

 clearly recognized appearance of the rediae is a miilberry-like cluster of cells, over 

 which a structureless membrane is soon foimed, while the pharynx and 

 other organs of the redia are produced in the cluster. There are 

 usually several of these germs in each cyst. This is a very character- 

 istic stage in the life history of the Trematoda ; the embryo is converted 

 into a bag, in which the germs of a new generation of individuals 

 originate and are confined until far advanced in their development ; 

 the body of the parent is converted into a temporary prison-house for 

 the progeny. The sporocysts of one species or another may be found 

 in nearly every snail ; many kinds are bizarre in shape, and all offer 

 the curious spectacle of the living germs squirming about and nearly 

 filling the Tihole of the cyst, their common parent. In the cysts of 

 Distomiim hepaticum there is usually one redia, less frequently two, 

 nearly ready to leave the sporocj'st, with two or three geims of medium 

 size, and several small ones. When ready to leave the sporocyst, the 

 redia by its own motion makes a forcible exit by rupturing the walls 

 confining it. The free rediae force their way through the tissues of 

 the host, and are found especially in the liver. They 

 increase in length, to 1.3 mm. or 1.6 mm. ; a sort 

 of collar is formed meanwhile a little behind the 

 pharynx. In other respects, except that they have a 

 digestive tract, which is wanting in the cysts, the 

 redise resemble the sporocysts in structure ; their most 

 important new feature is the bu'th opening at the side 

 of the body just behind the collar, which permits the 

 exit of the new brood developed within the redia. 

 The germs develop similarly to those of the sjjoro- 

 cj'sts, but are more numerous. Sometimes they pro- 

 duce a second genei-ation of rediae, probably as long as the weather 

 continues warm, but sooner or later, usually when cooler autumn 

 weather begins, there come redise, which produce a new stage, the 

 cercaria, in the series of metamorphoses. The cercaria is *^e typical 

 larval fluke, and is easily recognized from its appearance, which re- 

 minds one of a tadpole, as is shown in Fig. 171 ; it has a large body nearly as broad 

 as long, and flattened, with a long round tail. In the interior of the body one can 



Fig. 170.— Eedla 

 of Vistoma. 



Fig. 171.— Cercaria 

 of IHstoma. 



