ON THE ANATOMY OF NAUTILUS POMPILIUS. 193 



in a free concave edge. In the sexually mature animal the 

 much coiled-up spermatophore mass occupies the cavity of the 

 sac bending round the edge of the septum. The internal 

 surface of the sac-wall is smooth to the naked eye, while a low- 

 power lens discloses the existence of minute glandular-looking 

 ruga3 running on the whole parallel to the axis of the sperma- 

 tophore mass. 



At its anterior inner corner the cavity of Needham's sac 

 passes into the penis. This is a somewhat cylindroidal, flat- 

 tened structure about 10 mm. in length and 8 mm. greatest 

 breadth, attached to the body- wall within the mantle-cavity. 

 Its walls are thick and muscular, and its cavity is divided by a 

 sagittal longitudinal septum, which does not extend quite to 

 the tip of the organ, into two moieties. Of these it is only the 

 right with which the sac of Needham communicates, the left 

 being (as will appear later) connected merely with a peculiar 

 blind sac. The right penial cavity is somewhat semi-pyriform, 

 becoming narrower distally. Its lining is thrown into large, 

 smooth, glandular-looking rugae, which anastomosing with one 

 another form a kind of raised network with elongated meshes. 

 Outside this lining is the muscular coat about 1"5 mm. thick 

 and largely composed of radial fibres. The muscular layer is 

 traversed by an extensive system of blood-spaces. This is most 

 developed towards the " posterior " end of the penis. It forms 

 a distinct layer near the outer surface of the organ, but its 

 spaces also, though less conspicuously, ramify hither and thither 

 in the general substance of the muscle. 



The left penial cavity is cylindroidal in form, and its 

 diameter only about half that of the right cavity at its widest 

 part. The inner surface of its wall is also thrown into folds ; 

 but these are mainly longitudinal, parallel, and do not anasto- 

 mose to the same extent as do those of the right cavity. The 

 lining-tissue is of a less deep colour and less glandular-looking ; 

 the muscular wall is thinner, and the cavernous layer is also less 

 developed. 



At its " posterior" end, about the level of the point at which 

 the right cavity becomes continuous with the sac of Needham, 



