THE CEPHALIC SOMITES. 155 
occupied only by soft or imperfectly calcified cuticle, 
which, on each side of the hinder part of the mouth, 
passes into one of the lobes of the metastoma (mt). At 
the base of each of these lobes there is a calcified plate, 
united by an oblique suture with another, which occupies 
the whole length of the lobe and gives it firmness. The 
soft narrow lip which constitutes the lateral boundary 
of the oral aperture, and lies between it and the man- 
dible, passes, in front, into the posterior face of the 
labrum (Ib). 
In front of the mouth, the sternal region which apper- 
tains, in part, to the antenne, and, in part, to the man- 
dibles, is obvious as a broad plate (I7Z), termed the 
epistoma. The middle third of the posterior edge of the 
‘epistoma gives rise to a thickened transverse ridge, with 
rounded ends, slightly excavated behind, and is then 
continued into the labrum (Jb), which is strengthened by 
three pairs of calcifications, arranged in a longitudinal 
series. The sides of the front edge of the epistoma are 
excavated, and bound the articular cavities for the basal 
joints of the antenne (3); but, in the middle line, the 
epistoma is continued forwards into a spear-head shaped 
process (figs. 39 and 40, ZZ), to which the posterior end of 
the antennulary sternum contributes. The antennulary 
sternum is very narrow, and its anterior or upper end runs 
into a small but distinct conical median spine (fig. 40, t.). 
Upon this follows an uncalcified plate, bent into the form of 
a half cylinder (Z), which lies between the inner ends of 
