Chap. XVII.] THE LIVER-FLUKE AND TAPEWORM. 329 



There are thus many chances to one against further development ; 

 hence the vast number of ova. If it meets with a Limnaeus 

 truncatulus it spins round upon its axis, head-papilla protruded, 

 alternately shortening and suddenly extending its body, and 

 thus bores its way through the soft tissues into the pulmonary 

 chamber. It now becomes quiescent. The external ciliated 

 cells become swollen by absorption of water, their cilia sticking 

 out stiffly from them. The body becomes elliptical, the eye-spot 

 breaks up (Fig. 97, D.), and the organism passes into the condition 

 of a sporocyst. 



The Sporocyst. This now increases in size until it becomes 

 6 mm. long, while the germinal cells within it increase in num- 

 ber, partly by division of the previously contained germinal cells, 

 partly by the multiplication and division of the cells of the 

 body-wall, some of which become detached. The cells within 

 the sporocyst thus pass into the morula stage (E., m.). One side 

 then becomes flattened, and the cells here appear to be invagin- 

 ated, so that a gastrula (g.) is constituted; but the walls of the 

 gastrula are in contact, so that there is no primitive digestive 

 cavity or archenteron. Each spore is now surrounded by a 

 delicate membrane, and becoming more quadrate in shape, passes 

 into the redia condition (E., re., and F.) At one end a number 

 of cells are arranged to form a spherical pharynx (F., ph.), 

 which leads into a blind digestive sac (di.). A little behind the 

 pharynx the surface of the body is raised into a ridge, forming a 

 ring (col.) surrounding the anterior end ; whilst near the oppo- 

 site end, two short processes grow out (p, pr'.). This secondary 

 embryo has now passed into the redia stage. 



The Redia. Within such a sporocyst as is shown in Fig. 97, E., 

 there are generally one or two rediaa ready to leave, and others 

 in a more or less advanced stage of development. When it is 

 about -26 mm. long the redia bursts through the sporocyst, the 

 wall rapidly closing up again after the exit. The active little 

 redia finds its way into the digestive gland of the Limnseus, and 

 then feeds upon that organ, the annular ring or collar (Fig. 97, F., 



