Trematode Parasites of British Birds. 255 



an average lengtli of •]2 mm., hut it varies very much with 

 the state of extension or contraction ; the diverticula are 

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

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

 ventral sucker. In ^p. pygmas^im they extend as far back 

 as the posterior border of tlie sucker. 



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

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

 small size, the basal diameter being only •013-'014 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 v^entral sucker, the vesicula seminalis, or 

 the right iitestinal diverticulum, as is the case in Sp. puf/' 

 m(Bum, where it is of much greater size. The uterus 

 occupies 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, 

 •02()-'024' X •011-'()14 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 s(>en. This is probably responsible for Brandes's 

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

 not nearly .'^o large as those in Sp. pygmceum. Tlie yolk- 

 glaiuls 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. daviforme approaches most 

 nearly to Sp. jryf/mcEum. 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; uterus more voluminous, enveloping the testes *. 



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

 Sp. darifonne is not, ns Bnuides lliought, the adult of the cere-aria 

 describcii ])y M'lnlosh from Carcinus mocnas (Quart. J. Micr. Sci. 

 1865, p. 20\ ') is the tact that this cercaria measures ox '2 mm. (approx.). 

 It must therefore be the larva of some larger spc.'cies. A mucli more 

 likely cercaria is that described by Miss Lebour (Northumberland tJea 

 Fish. Kept. 190o, p. 6) from Litforinn nu/is, if it is not an antecedent 

 ptage of the above. If is much s-maller in size ('25 mm.}, but it has 

 rather long intestinal diverticula. 



