122 A. P. THOMAS. 



developed cercarise are present are frequently very sluggish in 

 their behaviour, and the ring may then be relatively incon- 

 spicuous. In very young redise the outline of the body appears 

 to present a slight process on each side anteriorly, and without 

 the most careful focussing it is often impossible to see that 

 these are simply the optical expression of the collar, the tissues 

 of which are still so delicate that the ridge is flattened above, 

 and therefore, owing to the transparence of its substance, not 

 readily recognised.^ The function of the collar is to maintain 

 the shape of the body and to produce a firm basis upon which 

 the neck of the redia can be moved. I have observed a redia, 

 whilst the whole of its body behind the ring was at rest, stretch 

 forth its neck in such a way as to sweep a considerable area in 

 front, and thus be enabled to reach conveniently the tissues of 

 the snail upon which it was browsing. When disturbed the 

 neck was retracted and the pharynx drawn back close to the 

 collar. But although the collar has thus a supporting function, 

 there is no thickening of the structureless cuticle in it, such 

 as could be termed a definite skeletal structure. 



The excretory system is better marked in the redia than in 

 the sporocyst, and definite canals can be distinguished in the 

 body-wall. Sinuous longitudinal vessels, one on each side, have 

 been described in the redise of several other Trematodes, and 



1 Diesing (' Wien. Sitzungsberichte/ vol. xxxi, p. 248) Las described the 

 redia of Cercaria fallax as having two short processes situated anteriorly, 

 and two, of thrice the length, posteriorly. De Filippi (' Memorie della Reale 

 Accadeniia delle Scienze di Torino,' Ser. ii, tomo xviii, p. 207) has described 

 the redia of Cercaria tuberculata as having four lateral processes, two 

 anteriorly and two posteriorly. I have met with a species which appears to be 

 identical with Cercaria tuberculata, and in the redia recognised a collar. 

 The same writer has figured (ibid., vol. xvi, pi. i, fig. 13) in the young redia 

 of Cercaria coronata four processes ; the two placed in front are sliglitly 

 smaller than the two posterior, but otherwise they are drawn as if exactly 

 alike. Tiiere can be no doubt that in all these cases the structures described 

 as anterior lateral processes are simply the projecting borders of tlie trans- 

 parent collar, seen perliaps in the flattened redia. From comparison with the 

 descriptions given by these distinguished observers 1 was led in my first paper 

 ('Roy. Agricult. Soc. Jouru.,' 1881, p. 19) to similarly misiutcrpret the 

 correspoudiug projeclious iu the young redise of Fasciola hepatica. 



