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 Kp 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 {lb). 



In front of the mouth, the sternal region which apper- 

 tains, in part, to the antennae, and, in part, to the man- 

 dibles, is obvious as a broad plate (III), 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 (lb), 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 antennae (5) ; but, in the middle line, the 

 epistoma is continued forwards into a spear-head shaped 

 process (figs. 39 and 40, II), 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 ol 

 a half cylinder (/), which lies between the inner ends of 



