200 
BRITISH ENTOMOSTRACA. 
The thorax and abdomen are very distinct from each 
other ; the former being twice the size of the latter. The 
thorax is composed of four segments, the abdomen of six ; 
but, in the female, the second and third are so united as 
to appear only one. The last segment terminates in two 
lobes. The first segment of the thorax, to which the 
head is immoveably fixed, and with which it is perfectly 
consolidated, is much larger than any of the others, 
being equal in size to all the other three; the second 
and third are nearly equal in size, and the fourth is the 
smallest. 
The antennae consist of numerous articulations, amount- 
ing to twenty-six ; each articulation throws forward a seta, 
sometimes two. In the male (t. XXIV, f. 1) they are shorter 
than in the female, and they each possess a peculiar swelling 
about the middle of their length, followed by a sudden 
contraction, the first articulation of which forms a hinge- 
joint. If we throw the animal into a state of semi- 
asphyxia by placing it in a mixture of spirits of wine and 
water, we perceive these antennae turn round in a con- 
trary direction to what they ordinarily do when in their 
native element, and curving their extremities at this 
hinge ‘joint, they bend back the part beyond it upon the 
swollen portion. The antennules are divided into four 
articulations (t. XXIV, f. a ), each furnished with several 
setae ; the terminating one having six of unequal length. 
The mandibles (t. XXIV, f. b) are composed of three 
parts, body, neck, and palpiform branch. 
The body is of an ovoid shape, convex on the upper, 
and concave on the under surface, in which concavity is 
lodged its motor muscle. The neck is a sort of petiole, 
turned upon itself, and dilated at its extremity, in which 
are implanted six tolerably strong teeth. The palpiform 
branch consists of one ring, and two long filaments. 
A little behind the mandibles we see the anterior, or 
first pair of foot-jaws (t. XXIV, f.c), which are strong organs, 
each composed of a pretty large body, convex externally, 
and concave internally, giving origin to two processes, the 
