iNcisuRA (scissurella) lytteltonensis. 7 



position. But in the case of the posterior filaments, which 

 are eight in number and much longer than the anterior fila- 

 ments, the twisting of the axis has brought the ventral 

 surfaces into a dorsal position. Fig. 16 represents a section 

 through the anterior and fig. 17 a section through the posterior 

 row of filaments. Each is more or less quadrangular in out- 

 line, its lateral walls formed of long columnar cells bearing 

 long and fine cilia, which in contracted specimens appear to 

 interlock like the cilia of the ciliated discs of filibranch 

 Lamellibranchia. I do not think, however, that their function 

 is to hold the filaments together, but simply to create 

 currents over the surfaces of the filaments. Their inter- 

 locking is simply due to their becoming matted together 

 in consequence of the conti^action of the gill in spirit. On the 

 ventral surface of each filament is a band of very short cilia. 

 The dorsal edge of the filament bears no cilia externally, but, 

 as shown in the figures, is produced to form a peculiar bolster- 

 shaped swelling, which, as far as I am aware, has no analogue 

 in the gills of any other mollusc. This dorsal glandular 

 ridge, as I will call it, takes its origin from near the free 

 distal end of the filament, and extending along the dorsal 

 face of the latter is closely fused to it for the greater part of 

 its length, but on approaching the proximal end of the fila- 

 ment the glandular ridge becomes free and ends in a small 

 rounded projection. The ridge is traversed throughout its 

 length by a small ciliated canal, which makes no communica- 

 tion with the blood channel of the filament, but opens into 

 the mantle-cavity in the angle between the free proximal 

 extremity of the ridge and the filament. This communication 

 with the mantle-cavity, as seen in section, is shown in the 

 central filament in fig. 17. In the filament on the right hand 

 in the same figure the section passes through the middle of 

 the glandular ridge, and the ciliated canal is seen to be closed 

 in on all sides and to be situated near the ventral, i.e. the 

 filamentary side of the ridge. The same features are shown 

 in the ridge attached to the right-hand filament in fig. 18, 

 but in the case of the left-hand filament in this figure the 



