o16 A HISTORY OF RECENT CRUSTACEA 
pereon. The six segments of the pleon may be all 
articulated, or some or all of them fused together. The 
seventh segment or telson is scarcely ever free, but its in- 
dependent existence is indicated in the peculiar genus 
Phreatoicus, and in one tribe, the Flabellifera, its presence 
may generally be inferred from the attachment of the 
uropods high up on the sides of the terminal segment, 
while a few of the genera, as Paranthura, have it distinctly 
articulated. 
A pair of sessile compound eyes, remote or contiguous, 
are usually present, but may be entirely wanting. Occa- 
sionally they project from the head, but never on movable 
foot-stalks. 7 
The first antenne are never very elongate, and rarely 
have a secondary flagellum. The second antennz very 
seldom carry an exopod, the articulated scale so common 
among the Macrura. The upper lip usually forms a plate 
projecting from the top of the oral aperture over the cutting 
edges of the mandibles, and may have an inner plate lying 
parallel to the outer. The lower lip is bilobed, or forms two 
pairs of lobes, of which the inner pair is much the smaller. 
The mandibles are very variable, the dentate cutting edge, 
secondary plate, spine-row, molar tubercle, and three-jointed 
‘palp, being sometimes strongly developed, at others, some 
or even all of them disappearing. The first maxillz very 
rarely have a backward directed ‘palp.’ The second 
maxille have neither ‘ palp’ nor exopod; the outer plate 
is divided, the inner undivided. The maxillipeds, of which 
the first are in this and the next sub-order the only pair, 
often have an epipod on the first joint. This first joint as 
a rule stands free in each maxilliped, the pair not being 
fused together. The plate developed from the second 
joint is provided with coupling spines. For the limbs of 
the trunk it is difficult to find any characters that are at 
all constant through the sub-order. Dr. Boas in his defi- 
nition (1883) says that these seven pairs are strong running- 
feet with a spine at the point of the terminal joint; that 
the basal joint is always small, the second joint long, the 
third in the Tanaide smaller than those which follow, 
