aviAjS cestodes. 863 



thickness from the neck onwards. This charactei'istic of the 

 species is not unfrequently met with in tapeworms ; it is, 

 however, perhaps unusually sti'ongly marked in this worm from 

 (Edicne'nvus histriatus. 



The scolex is long and oval, and provided with four lai'ge 

 suckers which possess no armature of hooks. 



When the scolex and the rostellum are extended the suckers are 

 quite in contact laterally. They are then oval in form. The 

 rostellam is long and retractile ; when fully extended it is as long 

 as the rest of the scolex. The extremity is entirely muscular 

 with seveiul layei'S of stout fibres running both transversely and 

 longitudinally. But in spite of its gi^eat muscularity there are 

 no hooks. It has been recorded, and I myself have had the 

 opportunity of observing a case, that among tapeworms which 

 possess an armed rostellum, the hooks are occasionally absent. 

 But in the present species I have never found them in a single 

 individual. I am thus disposed to think that they are really 

 absent, and to rely upon this as one of the distinguishing 

 characters of the genus, which will be duly set forth later*. 

 Behind the scolex there is quite a distinct neck, in which there 

 is no trace of segmentation to be observed. The segments of 

 the body overlap in the usual fashion and are never greatly 

 elongated. The most posterior are only slightly longer than broad. 

 The last external character to which I draw attention is the 

 position of the genital 2iores, which are unilateral. In the ripe 

 and more elongated segments they lie towards the hinder margin 

 of each proglottid. These oiifices are strictly lateral. In the 

 narrow anterior segments the genital pores have a peculiar dis- 

 position. In horizontal sections through a consecutive series of 

 such proglottids the edges of the proglottids stand out for a con- 

 siderable distance laterally. The genital pores open on these 

 extensions, but on to the anterior mai'gin of each where it runs 

 parallel with the posterior margin of the lateral extension of the 

 proglottid in front. The orifices are thus completely concealed on 

 a lateral view. In more mature segments these latexal extensions 

 do not stand out in the same straight line with the rest of the 

 segment, but become bent backwards ; so that the anterior margin 

 becomes lateral. 



The structure of a proglottid is illustrated in text-fig. 142 

 (p, 864). The cortical layer is about as thick as the medullary 

 layer. The former contains two rows of bundles of longitudinal 

 muscular fibres; the number of fibres constituting each of the inner- 

 most bundles is greater than that of those of the outer. There 

 appear to be a larger number of bundles also in the inner row. 

 These longitudinal muscle-bundles do not extend outside of the 

 nerve-cord ; they are not at all closely arranged. The ivater- 

 vascidar system consists of two lateral tubes on each side, of which 

 the outermost, or venti-al, is several times larger in cross-section 



* Vide Infra, p. 877. 



58* 



