12 COMPARATIVE ANATOMY 



fibres. The eyes and anterior papilla persist, and the 

 nephridial flame-cells increase in number to about half-a-dozen 

 on either side. Immediately within the longitudinal layer of 

 muscles is an epithelium lining the cavity of the sporocyst, 

 and forming the greater part of the thickness of its body-wall. 

 The interior of the cavity is filled by a number of very clear 

 rounded cells, some of which are the germinal cells of the 

 larva; others are derived from the epithelium lining the 

 cavity. Occasionally a sporocyst will divide into two by 

 transverse fission. The germinal cells in the sporocyst 

 undergo division and form spherical masses or blastulae, which 

 pass through a gastrula stage, and eventually develop into 

 a second kind of larva known as a redia. 



A redia has an elongated cylindrical body with an anterior 

 mouth leading into a muscular pharynx, and a short sac-like 

 digestive tube or enteron. A little way below the level of 

 the pharynx, the body is thickened to form an annular ridge 

 or collar, in which muscular fibres are strongly developed. 

 At a distance from the posterior end equal to about one- 

 fourth the entire length of the body are two blunt conical 

 processes, situated close together on one side. The structure 

 of the body- wall of a redia is much the same as that of a 

 sporocyst, but there are no eyes, and the excretory system is 

 better developed, consisting of two groups of flame-cells on 

 either side, an anterior group just behind the collar, and a 

 posterior group near the blunt conical processes or feet. The 

 anterior region of the body in front of the collar is filled up 

 with muscle fibres and parenchymatous tissue, but posteriorly 

 there is a large body cavity, the walls of which are lined by 

 an epithelium, like that of a sporocyst. A few muscular 

 strands traverse the body cavity. The wall of the enteron is 

 formed by a single layer of clear nucleated cells supported by 

 a basement membrane. When a redia has attained a length 

 of about '25 mm., it begins to move about actively, and pre- 

 sently it ruptures the wall of the sporocyst and passes into 

 the tissues of the snail. Many redise are produced by a single 

 sporocyst, and they may be seen in all stages of development 

 within the body of the latter. 



The free redise force their way by muscular contractions 

 through the body of the snail, and enter various organs, 

 particularly the liver. In so doing, they destroy the tissues, 



