Trttnatode Parasites of British Birds. 219 



tliose previously noted, except in one respect, the relative 

 sizes of the suckers. This is by no means an easy matter to 

 decide, for, the suckers being very nearly equal, slight 

 contraction on the part of either is sufficient to cause it to 

 appear smaller. In addition, owing to the method of 

 preservation, it frequently happens that one or both suckers 

 are deformed, i. e., elongated in one direction or another. 

 From previous measurements it was concluded that the 

 ventral sucker was, if anything, slightly larger than the oral 

 sucker, thus agreeing with Jagerskiold's observations. A 

 further extended series of measurements has caused me to 

 alter that opinion. In a considerable number of cases I 

 have found the suckers almost exactly equal, and in all 

 other instances, except one, the oral sucker exceeded the 

 ventral to a greater or less extent. This is perhaps the most 

 essential difference between Spehtrema exceUens and Sp, 

 simile, Jilgersk., apart from the diversity in size. Other 

 possible features of distinction, such as the size of the 

 genital body, the length of the ductus ejarulatorius, and 

 the situation and shape of other organs will be referred 

 to later. 



In most respects my previous description is fairly accurate, 

 but several features, the importance of which I did not then 

 realize, Avere overlooked. In describing the shape, exception 

 was taken to JagerskiokVs term " biscuit-t'ormig." This 

 term is occasionally used by continental observers ; it is not 

 very accurate, although it may be apt enough as understood 

 by them. I preferred the term " club-shaped,'' but that was 

 in reference more to the outline than to the actual body- 

 form, in the same sense, I believe, as Brandes used the 

 specific name claviforme. I can imagine no satisfactory 

 general term to describe this shape, which is almost peculiar 

 to the genus. If the outline were made to revolve on its 

 long axis a club-shape would be evolved ; in reality, 

 however, there is a very considerable amount of dorso- 

 ventral flattening, particularly in the anterior part of the 

 body, which is extended, and the sides of which display a 

 tendency to curl up ventrally. The presence of the large 

 number of ova causes a bulging in the posterior part of the 

 bofly, which, moreover, has an elliptical or almost circular 

 outline. In describing Sp. pygmceum (Levins.), Odhner "^ 

 uses the term " keulen-formig'' and adds that in contracted 

 conditions the breadth increases gradually from before 

 backwards. This would involve a somewhat triangular 



* Fauna Arctica, iv. (2) pp. 31o-316. 

 Ann. & Mag. N. Hist. Scr. 7. Vol. xx. 17 



