LIFE HISTORY OF THE LIVER-FLUKE. 121 



whilst wandering througli the tissues of the host. They are 

 not situated on opposite sides of the body^ but are close 

 together on the same surface, and their bases may even be 

 connected by a low transverse ridge. They are directed out- 

 wards and somewhat backwards, their axes being usually 

 inclined at an angle equal to or rather less than a right angle. 



The body-wall has a similar structure in both redia and 

 sporocyst, so that it will only be necessary to describe the 

 points in which a difference exists. The muscle-fibres are far 

 more strongly developed, especially in the anterior part of the 

 body, so that the redise show considerable activity as compared 

 with the sporocyst. When the body is fully extended it may 

 have a length twice as great as when in a state of contraction. 

 If an example of the host be chosen, which has a clear and 

 transparent shell, and has had the greater part of its liver 

 consumed by the parasites, the rediae may be observed perform- 

 ing movements of elongation and contraction whilst still within 

 the living snail. 



In the collar or ring mentioned above the muscle-fibres are 

 strongly marked, and have a peculiar arrangement. The trans- 

 verse muscle-fibres appear to lie directly under the cuticle, and 

 are closer together at the sides of the ridge then near its most 

 convex part. The longitudinal muscle-fibres, however, do not 

 follow the curve of the surface, but stretch across from one to 

 the other side of the base of the ridge. Sometimes, when the 

 ring is strongly marked, the longitudinal muscle-fibres as they 

 pass forwards may spread out in a fan-shaped fashion before 

 they are finally inserted in the cuticle (fig. 16). The extent to 

 which the ring projects above the rest of the surface of the 

 body varies very greatly according to the size and condition of 

 the redia, and may be altered from time to time by the con- 

 traction of the muscle-fibres. It is greatest in those which 

 show the most active movements, least in those which are the 

 most passive. The smaller or half-grown rediae commonly 

 show the greatest activity, and in one of these I have observed 

 the ring so enormously developed that the diameter of the body 

 was almost doubled at this point. Those redise in which fully- 



