120 A. P. THOMAS. 



wound produced by the forcible exit of the redia is kept closed 

 until it has healed up. Meanwhile the development of the 

 remaining germs proceeds. Many of the nurse-forms of 

 trematodes are known to possess a special birth opening for 

 the escape of the brood, and even amongst the sporocysts sucli 

 an opening is present in those possessing a filiform shape; and 

 I have myself observed this structure in the sporocysts of 

 Cercaria gracilis. But in F. hepatica no such definite 

 opening can be detected in the sporocyst. 



2. The Free Redia. — The sporocysts of the liver-fluke are 

 found in the pulmonary chamber of the snail, or less abundantly 

 in the body cavity. But the free rediae force their way through 

 the tissues of the host, and wander into the other organs, and 

 especially into the liver. They are usually found, with the 

 digestive tract quite yellow from the remains of the snail's 

 liver-cells, with which it is filled. In thus forcing their way 

 through the tissues they necessarily inflict much injury on 

 their host, so much, in fact, that comparatively few snails survive 

 three weeks from artificial infection, and the majority, even of 

 these, die before the time when the cercariae are completely 

 mature. Thus, in the laboratory at any rate, the fluke-disease 

 is more fatal to the snail than it is subsequently to the sheep. 



The redia increases in size until it may reach the length 

 of 1'3 mm. to 1*6 mm. It has an elongated cylindrical 

 form (PI. Ill, figs. 12, 13), and at a little distance behind the 

 pharynx there is present an annular ridge or collar projecting 

 from the surface, the use of which will be explained below. 

 From this ring or collar the body tapers gently towards the 

 anterior end, which is abruptly truncated, and includes in its 

 centre the mouth. Behind the collar the body becomes a little 

 narrower, but then swells out gently again until it reaches the 

 middle of its length, from which point it tapers, at first almost 

 insensibly, and then more rapidly, the extremity being conical 

 with rounded apex. At a distance from the posterior end, 

 equal to about one fourth of the total length of the body, are 

 situated two short and bluntly conical processes, which serve 

 as rudimentary feet, and are no doubt of much service in 

 steadying the redia and preventing it from slipping backwards 



