282 D. J. SCOURF D ON THE 



rod is always divided into two almost equal portions, either by 

 a slightly projecting ridge (Figs. 2 and 3), or by a shoulder 

 (Fig. 4), or at least by a faint line (Figs. 5 and 6). It is true 

 that, owing to the extreme minuteness and delicacy of these 

 setae, I have not been able to satisfactorily make out the divi- 

 sion into two parts in some cases, but it can be demonstrated in 

 so many others that there seems little doubt that it is really a 

 constant feature. The importance of this character is that it 

 enables us to correlate with certainty the whole of the hyaline 

 rod, i.e., the whole of the external portion of the seta, with 

 the distal hyaline portion of very similar sense-hairs on the 

 antennae of other crustaceans, as for example Asellus aquaticus 

 (Fig. 7) and Astacus fluviatilis, for in these cases also the 

 division of the hyaline portion into two parts is present. 



Leaving the essential structure, a few words may next be 

 devoted to some of the principal variations noticeable in different 

 species. As regards the shape of the main, i.e., the hyaline, 

 portion of the seta, there is not much to be said, for in the vast 

 majority of cases it is little more than a straight or slightly 

 curved rod of nearly uniform diameter. In the case of Eurycer- 

 cus lamellatus (Fig. 4), however, the distal half of the rod is only 

 about half the diameter of the proximal half, and in Acanthole- 

 beris curvirostris (Fig. 6) the sudden diminution in diameter 

 towards the tip produces rather a characteristic appearance. 

 The tip of the seta itself may be either rounded, truncated, or 

 produced into two little points, although in some cases the latter 

 appearance is possibly due to an optical illusion caused by the 

 highly refracting pellet (see Figs. 4 and 4a). In other cases, 

 however, e.g., Acantholeberis (Fig. 6) and Macrothrix, the two 

 projections from the tip are so large as to form veritable hooks, 

 about which no question of optical illusion can arise. The pellet 

 of chitin at or quite near the tip of the seta, although such a 

 noticeable feature, is difficult to examine on account of its small 

 size and highly refracting nature. It usually appears as a simple 

 spherical granule, but sometimes gives one the impression that it 

 may be a ring, and in yet other cases I am inclined to think that 

 it has a cup-like form (Fig. 3a) or at least that it forms a solid 

 button at the bottom of a cup-like depression in the tip of the 

 seta (Fig. 4a). But the most variable part of the seta is 

 undoubtedly the chitinous basal piece to which the hyaline rod 



