2l8 



DISCOVERY REPORTS 



setal armature is the same, and their exopodites take the place of the exopodites and 

 epipodites of Nebalia. It is legitimate to deduce that Nebaliella is also a filter-feeding 

 organism and that the principle of its filter mechanism is the same as that of Nebalia. 

 Nebaliella, like Nebalia, is undoubtedly a mud-living form. In fact, the anterior 

 portion of the Discovery specimen was covered with a layer of mud which had to be 

 removed before its limbs could be studied. The shape of the rostrum and eyes and the 

 armature of the third and fourth joints of the antenna, which, as I have explained, are 

 the other characteristics of the genus, are, I believe, adaptations to this burrowing mode 

 of life. In Nebalia I suggested (1927, p. 356) that the rostrum functions in controlling 

 the current entering the filter chamber. This has been carried to extremes in Nebaliella 

 and, the animal being apparently blind (Thiele, 1904, p. 4), the eyes have become modi- 



Fig. 6. Nebaliella extrema. Side view. 



fied in structure and incorporated into this rostral controlling mechanism. Briefly put, 

 the eyes and rostrum together form a plough which forces the mud to the sides and so 

 prevents the filter mechanism becoming choked as the animal forces its way through the 

 mud by means of the powerful claws on the antennae. 



The shape of the rostrum and eyes has been described in detail by Thiele (1905, 

 p. 61). They are figured in Fig. 7 in lateral aspect and when viewed obliquely from in 

 front and below. It will be seen that the rostrum consists of a median keel with lateral 

 flanges posteriorly. The eyes are excessively thin scythe-shaped plates. The planes of 

 the eyes are approximately at right angles, each projecting antero-laterally. Thus, in 

 Fig. 7 B, the right eye, which is being viewed in the direction of its plane, shows its 

 thinness, while the left shows its outline. The eyes articulate below the lateral flanges 

 of the rostrum, the convex anterior margin extending up to a point immediately under- 

 neath, and slightly median to, the outer edge of the flange. The articulation is thus 

 covered dorsally by the rostral flange. 



The concave posterior margin of the eye is produced into a heel near its upper limit 



