326 Merrill—Structure and Affinities of Bunops Scutifrons. 
are never all present on the same leg, however. On the inner 
side of the stem there is a vesicular process, the gill sac, which 
is always present. This gill sac is small in the Macrothricidce 
and is usually oval or kidney-shaped. In Bosmina and the 
Lyneeidoe it is drawn out into a long process. 
Immediately above the gill sac there is in many Cladocera a 
delicate ciliated lobe or extension. It is found only in the 
more posterior legs. On the outer side is a process which 
ideally is flattened and rectangular, but usually becomes trian¬ 
gular in shape. It bears setae and cilia, and is frequently 
coalesced with the inner branch cf the stem. This is the maxil¬ 
lary process. It is sometimes absent in the first leg, but usu¬ 
ally present in all the others. The “setae terminales stipitis” 
lie below the maxillary process and consist of a stout ciliated 
seta on each side. 
There are two branches attached to the distal end of the stem. 
The inner branch is lobed and forms the working part of the 
leg. The outer is membranous. In the anterior legs, the inner 
branch is most strongly developed. In the posterior ones the 
outer is the larger. 
Bunops scutifrons has five broad leaf-like legs. They conform 
in general to the type of structure of the Macrothricidce , show¬ 
ing differences in detail from those of any other genus. 
FIRST LEG. 
Plate XV, Fig. 1. 
As in all the family the first leg is very large and broad, 
with hooks and setae capable of projecting beyond the edges of 
the valves. Starting from its point of attachment, it runs for¬ 
ward parallel with the labrum, and then bends down abruptly, 
forming a sort of knee. Above the bend there is a tuft of deli 
cate bristles, and a little within the margin above the gill sac 
are two strong, posteriorly curved setae. The gill sac ( g) is 
the only process present upon the stem. The maxillary process 
seems to be entirely lacking. The distal portion of the leg con¬ 
sists of two branches, the ramus interior and the ramus ex¬ 
terior. The latter is represented only by a slight projection 
