DIAPTOMUS. 
221 
by five of different lengths. In the male, the right an- 
tenna alone has the swelling and hinge-joint, which cha- 
racterise the sex. This joint is formed in the same manner 
as that in the Cyclopidse. 
The antennules (t. XXVI, f. 2 a) are of considerable size, 
and bifid ; two branches of unequal length, arising from a 
common footstalk. The shorter of the two can be moved 
backwards or forwards at the pleasure of the animal, and 
consists of six articulations, the first of which is stout, and 
inserted into the common footstalk, and has four stout 
setae springing from its edge. The second, third, fourth, 
and fifth are very short, and each is furnished on its edge 
with a stout seta ; while the last is of considerable length 
and is provided at its extremity with three long setae. 
The longer branch is composed of three joints. The first 
is articulated with the body of the animal, and the third 
terminates in several long filaments, which have a joint in 
the middle of their length, adding much to their supple- 
ness. These organs the little creature puts into rapid 
action, and so causes a regular whirlpool in the water, 
which attracts every object into its centre, and guides 
them into its mouth. 
The labrum or lips consist of two small, somewhat 
foliated-looking bodies, almost transparent, which, when 
any object fit for food passes into the mouth, are seen to 
separate from each other and open a passage for it. 
The mandibles (f. 2 b) resemble those of the Cyclops 
(already described), but have the palpiform branch much 
larger and bifid. 
The neck terminates in a horny acute point, under 
which are six small teeth on the same plane. 
The foot-jaws are three pairs in number. The anterior 
or first pair (f. 2 c , d) consists of a broad, heart-shaped 
plate, with a smaller one of the same form attached to its 
upper edge, both having numerous setae springing from 
them, in the larger consisting of four on one side, and 
eight on the other ; while a third, or smaller plate, bifid, 
and equally beset with a number of hairs, springs from 
