Trematode Pcaxisites of British Birds. 255 



ail average lengtli of "12 mm., hut it varies very much witli 

 the state of extension or contractiou ; the diverticula are 

 almost constantly '08 mm. long. The latter are dilated, 

 very wide apart, and do not nearly reach the level of the 

 ventral sucker. In Sp. pijgm(eum they extend as far back 

 as the posterior border of the sucker. 



The genital body is situated close to the left side of tlie 

 ventral sucker, on a level with its anterior half. It is of 

 small size, the basal diameter being only •013-'0]4 mm. 

 The vesicula seminalis is transversely oval and lies sym- 

 metrically in the intestinal bifurcation, a considerable 

 distance in front of the ventral sucker. The ductus ejacula- 

 torius is slightly sinuate. The ovary is also correspondingly- 

 far forward. It is an irregularly rounded body and is not 

 contiguous with the ventral sucker, the vesicula seminalis, or 

 the right ii.testinal diverticulum, as is the case in Sp. pi/n. 

 niiBum, where it is of much greater size. The uterus 

 occTipies most of the posterior part of the body and extends 

 forward on each side to the end of the intestinal diverticula. 

 The ova are fairly numerous and of comparatively large size, 

 •O20--024- X 'Oll-OM mm. They are colourless on the 

 right, bright yellow on the left. The uterus completely 

 envelops the testes and yolk-glands, so that these organs can 

 hardly be seen. This is probably responsible for Braudes^s 

 mistake. The testes have much the same situation, but are 

 not nearly so large as those in Sp. pygmaum. The yolk- 

 glands are behind and ventral to the testes, but division into 

 lobes, if such, indeed, be their condition, is not distinctly 

 marked. 



From the foregoing it is apparent that in point of size 

 and possibly on the whole, Sp. cluviforme approaches most 

 nearly to Sp. 'pycjnmum. Several differences have already 

 been indicated, but they may be briefly recapitulated as 

 follows : cuticular scales are more extensive ; suckers are more 

 unequal and the ventral sucker is further back ; surrounding 

 structures are thus relatively further forward ; intestinal 

 diverticula shorter compared with oesophagus ; genital organs 

 all smaller J uterus more voluminous, enveloping the testes *. 



* A conclusive proof, which I previously neglected to adduce, that 

 Sp. claviforme is not, as Brandes tliought, tlie adult of the cercaria 

 described by M'Intosh txom Carcinus mcenas (Quart. J. Micr. Sci. 

 ]865, p. 201) is the fact that this cercaria measures -ox '2 mm. (appro.x.). 

 It must therefore be the larva of some larger spi'cies. A much more 

 likely cercaria is that described by Miss Lebour (Xorthumbfrland Sea 

 Fish. Rept. 1905, p. 6) from Littorinn rudis, if it is not an antecedent 

 stage of the above. It is much smaller in size (-20 mm.}, but it has 

 rather long intestinal diverticula. 



