516 



ZOOLOGY 



SECT. 



area, the articular membrane, the chitinisation of which is less pro- 

 nounced than that of the rings themselves. By this arrangement 



the segments 

 are freely mov- 

 able upon one 

 another in all 

 directions, the 

 articular mem- 

 branes acting 

 as joints. The 

 last or anal seg- 

 ment is pierced 

 by the terminal 

 anus (Fig. 431, 

 an.). 



The ventral 

 surface of the 

 head is formed 

 by a flattened 

 sub-frontal plate 

 (Fig.429,5./.^.), 

 continuous 

 marginally with 

 the carapace. 

 The posterior 

 edge of this 

 plate is convex 

 backwards, and 

 is produced in 

 the middle line 

 into a shield- 

 shaped process, 

 the labrum or 

 upper lip (Ibr.), 

 which over- 

 hangs the 

 mouth. From 

 the sub-frontal 

 plate also arise, 

 on each side, 

 two delicate 

 processes, the 

 innermost 

 called the anten- 

 nule (ant. 7), the 

 outermost the 

 antenna (ant. 2) : these are the first two pairs of appendages. The 



