XI 



PHYLUM ARTHROPODA 



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small spines : each ring slightly overlaps its successor, and is 

 connected with it by a narrow area, the articular membrane, the 

 chitinisation of which is less pronounced than that of the rings 

 themselves. By this arrangement the segments are freely movable 

 upon one another in all directions, the articular membranes acting 

 as joints. The last or anal segment is pierced by the terminal 

 anus (Fig. 424, an.). 



The ventral surface of the head is formed by a flattened sub- 

 frontal plate (Fig. 422, s.f.pl.), con- 

 tinuous marginally with the cara- 

 pace. 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 antennule 

 (ant. 1), the outermost the an- 

 tenna (ant. 2) : these are the first 

 two pairs of appendages. The 

 third pair consists of two strong, 

 toothed bodies of a deep brown 

 colour, placed one on each side of 

 the mouth, and called the man- 

 dibles (md.). The remaining ap- 

 pendages form two rows of deli- 

 cate leaf-like processes, attached 

 to the segmented portions of the 

 body, and overlapping one another 

 from before backwards : their num- 

 ber varies from forty to nearly 

 seventy (th.f^abd.f.). 



Appendages.- -The antenmde 

 (Fig. 423, 1) consists of a bent 

 rod bearing delicate chitinous 

 bristles or seta; at its tip, and presenting, at the bend, a joint, due 

 to the presence of an articular membrane. The appendage is 

 thus made up of two podomercs or limb-segments, movably 

 articulated together. Its function is probably tactile. 



The antenna (#) is absent in some species both of Apus and 

 Lepidurus : in A. cancriformis it is a very delicate hook-shaped 

 unjointed structure, probably functionless. As we shall see from 

 the study of development, it is a vestigial organ. 



The mandible (3} is also an unjointed appendage. It has the 

 form of a deeply concavo-convex plate, strongly chitinised, and pro- 



YOL. I M M 



FIG. 422. Apus glacialis, ventral 

 aspect, abd. f. abdominal feet ; ant 1 . 

 antennule ; an ft. antenna ; Ibr. labrum ; 

 md. mandible ; mx. first maxilla ; ov. 

 aperture of oviduct ; .*. /. />!. sub-frontal 

 plate ; xli. yl . shell-gland; th.f. thoracic 

 feet ; th.f.l, first thoracic foot. (After 

 Bernard.) 



