TAPEWORM FROM APTERYX. 91 



muscles have a horizontal direction, i. e. their axis is at right 

 angles to the long axis of the entire organ. But when the 

 rostellum is protruded they become vertically disposed 

 (figs. 18, 20). 



For a long time I was unable to determine the mode of 

 insertion of these muscles, but in good longitudinal sections, 

 and even afterwards in old preparations, I was able to 

 recognise an extremely thin membrane (fig, 19, j) that, in a 

 retracted state, is folded within the acanthophore. 



In fact, the free distal extremity of the rostellum is in- 

 vaginated as a delicate membrane ; and when the apparatus 

 is fully everted this membrane is forced outwards, and 

 assumes a convex foi-m (fig. 20, j), taking with it, of course, 

 the insertions of the radial muscles. 



It is very difficult to determine this invaginated membrane ; 

 for some time I believed that the terminal wall was double, 

 and that the membrane was capable of movement to and 

 fro, as implied by Studener (vide Bronn). I thought I could 

 detect the aperture of invagination at the apex, and then 

 again I could not make it out in the other case or in the 

 specimens mounted whole. But a comparison of a series of 

 camera outlines, representing* every stage of protrusion, as 

 well as a renewed study of several states of protrusion, 

 showed that the circle o£ booklets increased its diameter on 

 protrusion ; and I feel confident that the explanation of this 

 lies in the facts above recorded. 



Neither this membrane nor the muscles are directly con- 

 nected with the booklets. 



The cavity of the rostellum, or, to be more particular, of 

 the reservoir, is occupied by a granular material, in which 

 numerous nuclei are recognisable, but without cell outlines 

 (figs. 16, 19, 20, t). In certain preparations a fibrillation of 

 this material is apparent ; these fibrill^e, however, are not 

 muscular ; they react to stains quite difi^erently, and do not 

 stain at all ; I am inclined to think that they are due partly 

 to the arrangement of granules in rows, and possibly to some 

 coagulation effect. This granular material is very compress- 



