546 Marsh—Structure of the Larva of Epischura Lacustrts. 
which this figure was drawn could hardly be distinguished, in 
most respects, from one which had reached complete maturity. 
The asymmetry, then, seems to come in quite suddenly, and in 
the last stages of the development. 
The furca of the mature Epischura is armed with three broad 
setae, an external spine, and a weak and inconspicuous internal 
seta. In the one segment stage, the three setae are much 
weaker, the internal seta is considerably longer, and there is, 
in addition, a lateral seta about midway of the length of the 
furca. The furca of Epischura in this stage differs very little 
from that of Diaptomus at the same degree of development. 
Diaptomus also has a lateral seta, but it is longer than that of 
Epischura , and is situated nearer the end of the furca. The 
lateral seta does not appear in the succeeding stages of Ep¬ 
ischura , the armature in the three segment stage differing very 
little from that in the mature animal. In Diaptomus the six 
setae develop to nearly the same length and size, the lateral 
seta having moved down nearly to the end of the furca. In 
Limnocalanus the lateral seta remains upon the side of the 
furca, and the sixth, or internal seta is very weak. In Ep¬ 
ischura the lateral seta disappears, the third, fourth, and fifth 
are largely developed, the second becomes a broad spine, and 
the sixth a short and weak seta. In Cyclops it is the third and 
fourth setae which are commonly the most highly developed, 
the second and fifth sometimes nearly equalling them, but in 
most cases being reduced to weak and short setae or spines. 
DEVELOPMENT OP THE MALE FIFTH FOOT. 
The fifth foot of Epischura lacustris consists of two parts, in 
which there is no clear evidence of the division into exopodite 
and endopodite. (Plate XIII, Fig. 5.) The right foot consists of 
two segments — the outer is triangular in shape, with a some¬ 
what spatulate tip, and is always flexed upon the first segment. 
The left foot consists of three segments: the first has a long 
horn-shaped lateral process, which is as long as the other two 
segments; the second and third segments are concave on their 
inner margins; and the apical segment is armed with fine hairs 
upon the concave margin and with small spines at the tip. 
