102 



LOWER tN VER TEF.RA TES. 



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

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

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

 (.liaugcs into a new form, the nurse or sporocyst, within which arise the 

 gei'ins or sj)ores producing new iudiviiluals. The outer ciliated cells 

 swell up, and are finally cast off. The eniluyn then becomes a little 

 bag or cyst, at one end of which the pigmented 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 

 Fir,, ifin.— Cyst in autuum twice that time mav be necessarv. These cysts sometimes, 



of Distomn. _ ' ' . . . 



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

 phcuonienon is more frequent. 



The next larval forms, tlie reili:e, -aw develojied within the sporocyst. The first 

 clearly recognized aiijiearance of the redi.e is a mulberry-like cluster of cells, over 

 which a structureless membrane is soon formed, 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 veiy 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 develojiment ; 

 the l)o<ly of the parent is converted into a temporary prison-house for 

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

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

 the curious spectacle of the living germs squirming about and nearly 

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

 Distomuin hepatictmi there is usually one redia, less frequently two, 

 nearly ready to leave the sporocyst, with two or three germs 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 fi-ee rediiB 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 s(n-t 

 of collar is formed me:mwhile a little behind the 

 )iharyn\-. In other resjieets, exce]it that they have a 

 digestive tract, which is wanting in the cysts, the 

 rediie resemble the sporocysts in structure ; their most 

 important new feature is the birth opening at the side 

 of the Ijody just ln'hind the collar, which permits the 



exit of the new In 1 <le\el(iped within the redia. 



The germs develop similarly t(i those of the sjtoro- 

 cysts, l)ut are more numerous. Sometimes they pro- 

 duce a second generation of rediiv, probably as long as the weather 

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

 weather begins, tliei-e come redise, which produce a new stage, the 

 '^'o. iTi — Cercaria ccrcaria, in the series of metamorphoses. The cercaria is the typical 



ot Distoma. . . . ' . / ' 



laj-val 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 



L 



