No. I.] SPHYRANURA OSLERF. 1 3 



where, but that it has apparently undergone a chemical change, 

 assuming a bluish-green stain with iodine-green, which is not 

 present elsewhere. As the wall of the sucker is itself destitute 

 of contractility another arrangement exists for modifying the 

 shape of the cavity. Its wall is really divided into three con- 

 centric zones, which, by special extrinsic muscles, can be worked 

 independently. The two circular lines which separate these 

 zones are marked by an infolding of the investing membrane, 

 which forms a sort of joint, permitting the independent move- 

 ment of the zones. These lines mark off the concentric bands 

 seen from the ventral aspect, while the doubly contoured line 

 surrounding the central booklet is the expression of the invest- 

 ing membrane, lining the funnel-shaped perforation in the 

 wall of the sucker occupied by the booklet, and the soft parts 

 on which it rests. 



The movements of the sucker are best seen when the worm 

 is uncovered, and able to curl round the edges of the caudal 

 lamina. One thus has a side view of the sucker, and readily 

 sees the independent movements of the second and third tiers 

 in altering the shape of the cavity. These movements are 

 effected by extrinsic fibres in the substance of the caudal lamina, 

 which are partly disposed equatorially near the aperture of the 

 sucker and meridionally over its convexity, while more distinct 

 bundles penetrating the dorsal wall of the caudal lamina (Fig. 3) 

 are attached to the second and third tiers of the sucker and to 

 the central booklet. Some of these fibres appear to penetrate 

 the supportive framework of the sucker. Other muscular 

 bands which act upon the suckers will be referred to under 

 the musculature of the caudal lamina. 



The Musculature. 



We have little to note here as to the general arrangement of the 

 musculature, except that the diagonal fibres so abundantly pres- 

 ent in the larger Distomes are hardly represented here, so that 

 we have merely to do with an external circular, a deeper longi- 

 tudinal layer of fibres, and the somewhat scattered parenchyma 

 fibres. The longitudinal fibres are much more developed on 

 the ventral than on the dorsal aspect; instead of forming there 

 a single layer they are disposed in two strong sheets, right and 

 left of the middle line, this local development being due to the 



