340 Merrill—Structure and Affinities of Bunops Scutifrons. 
gans Lathonura has the most primitive form. All of the setae 
are short, plumose, jointed and similar in character. Ophry- 
oxus and Bunops come next with very similar antennae, having 
a stiff but weak basal seta, practically free from spinules. 
Macrothrix, Acantholeberis and Streblocerus show a gradual en¬ 
largement of this seta, until in Drepanothrix it becomes extra¬ 
ordinarily heavy and sabre-like. 
There are two prominent modes of swimming in this family. 
Lathonura, Macrothrix, Acantholeberis and Streblocerus move by 
sudden jerks with intervals of rest. Ophryoxus and Bunops 
have a steady paddling motion of the antennae, giving a motion 
quite different Irom the jerks of Lathonura . This habit is car¬ 
ried to the extreme in Ilyocryptus , which makes the most patient 
and persistent efforts to move when held down in a live-box. 
Drepanothrix swims as little as may be, but scrambles about on 
the bottom while practically resting its heavy seta on the vege¬ 
table debris over which it scrambles. It does not seem to swim 
as freely as Ilyocryptus . 
LEGS. 
We find three types of Macrothricidoe in respect to the number 
of legs. Lathonura and Grimaldina have four pairs; Guernella , 
Macrothrix, Streblocerus, Drepanothrix and Bunops have five 
pairs, while Ophryoxus , Acantholeberis, and Ilyocryptus have 
six. 
In the comparative study of the legs of Bunops, I have used 
Lund’s paper for the genera Bosmina, Lathonura, Macrothrix, 
Ilyocryptus and Acantholeberis, and my own dissections and un¬ 
published drawings of Prof. Birge, for the genera Ophryoxus and 
Drepanothrix. 
The most striking peculiarity of the legs is the possession of 
the hooks on the first foot. In this, Bunops agrees most closely 
with Ophryoxus. That genus also has the first foot armed with 
two strong hooks of'unequal size on the second division of the 
leg. Drepanothrix has one such hook, but much smaller and 
weaker. In that genus, however, the branchial sac of the 
first foot is much elongated and closely resembles the corres- 
