358 H. s. PRATT, 



have been wrongly intrepreted by some authors to be pore-canals. 

 In fresh and uninjured animals, according to Looss, these structures 

 never appear. Blochmann (1895), too, has shown that solid bodies 

 which have often been taken for nuclei in the cuticula of Trematodes 

 are often the end-organs of sensory nerve-cells. 



The integument of the appendix of the worm under discussion 

 differs in certain important respects from that of the trunk. In con- 

 siderably more than half of the individuals examined, it is a typical 

 Trematode-cuticula ; it is a continuation of the cuticula of the trunk 

 and not different from it in structure or thickness. Fig. 7, PI. 26, 

 represents such a case. This shows the retracted appendix with its 

 extremity thrust partly back, as if it were just being evagiuated when 

 the animal was killed: the point where its cuticula joins that of the 

 trunk is not marked in any way. In a large number of individuals, 

 however, the appendix is covered with a columnar epithelium, on the 

 outer surface of which is a fine cuticula. The height of the cells of 

 this epithelium is considerably greater in the invaginated (PI. 26, 

 Fig. 5) than in the extended appendix (PI. 25, Fig. 3): in both cases 

 the outer ends of the cells are convex in outline, so that the whole 

 cellular layer does not present a smooth and even outer surface. 

 The nuclei are large, and in the extended appendix are round; in 

 the retracted appendix, however, they are elongated; this latter fact, 

 as well as the greater height of the cells themselves in the retracted 

 appendix being due, no doubt, to the compressed condition of the 

 epithelium in this position. The height of these cells in the extended 

 appendix is 0,01 mm, in the invaginated appendix it is 0.013 mm. 



Beneath the epithelium is a thick membrane which is a con- 

 tinuation of the cuticula of the trunk and not different from it in any 

 important respect (PI. 25, Figs. 3 and 4 s.mem). Like the cuticula 

 it is composed of two layers which react towards staining reagents in 

 the same manner as the two layers of that membrane: its thickness 

 in the invaginated appendix is also the same as that of the cuticula; 

 in the extended appendix it is somewhat less. I have noticed that 

 vesicular structures occasionally occur in this subepithelial membrane. 

 In one animal the entire outer surface of the membrane, that which 

 abuts the epithelium, appears studded with minute but extremely re- 

 gular, cup-shaped vesicles of which about six would appear in a section 

 beneath the base of a single cell. In another animal the peculiar 

 palisade-like structure already mentioned appeared in this membrane 

 (PI. 26, Fig. G). The cup-like vesicles are exactly similar to those 



