AJS^D THE SUEFACE-FILM OF "WATER. 7 



extremely delicate membrane or series of hyaline scales is sup- 

 ported by the setae, very little more can be made out by exami- 

 nation in this position. To really learn anything of the structure 

 and arrangement of the setae, they must be viewed from the 

 front, and even then, owing to the thickness and dark colour of 

 the animal, special care is needed to demonstrate the finer 

 details. The setse arise from a definite flattened area running 

 down the greater portion of the margin of each valve (PI. II. 

 fig. 1). This area, which is bounded by the edge of the valve and 

 a slight ridge almost parallel to it, represents, no doubt, an 

 original line of hexagonal shell-markings similar to those covering 

 the general surface of the valves. The setse, it will be noticed, 

 are not alike all the way down, but are divided into three distinct 

 series. 



The anterior series (PL II, fig. 2) usually comprises about 

 twelve apparently tubular setse arranged in a single row. The 

 line of their bases occupies a median position on the flattened 

 area for the posterior half of its length. ; but anteriorly it curves 

 towards the edge of the valve. The first few setse are directed 

 forward, and at the same time are strongly curved inward; 

 but as the series is followed backward the amount of this cur- 

 vature decreases, and the general direction of the setse is also 

 gradually changed, so that tbose of the posterior half of the 

 series come to point outward. From the base of all except the 

 first two or three setse, a branch is given off which turns in the 

 opposite direction to that pursued by the main branch ; and in 

 nearly all cases each of the branches further gives rise to a little 

 subsidiary outgrowth directed forward. It sometimes happens 

 that the inner branch is widely separated from the other at the 

 base, and then the anterior series practically consists of a double 

 row for a large portion of its length. The last seta is, however, 

 always single, and possesses a short peduncle anterior to its 

 bifurcation. Besides the coarser branching, each seta produces 

 near its distal extremity, and the last also along its posterior 

 margin, a number of exceedingly fine processes, which are usually 

 grouped in bundles of three or four. They can be most easily 

 observed on the last seta (PI. I. fig. 4) ; and in this case also a 

 slight break in the edge of the seta can occasionally be seen at 

 their point of origin. Prom their excessive delicacy it is still 

 uncertain whether these are simple hair-like outgrowths or only 

 corrugations in a hyaline membrane supported by the setae. 



