THE ARTHROPODA 93 



beneath, but the anterior portion is firmly fused to and is 

 continuous with the head and body region in front of the 

 nuchal grooves. The posterior part of the shield is, in fact, 

 a reduplication of the chitinous cuticle of the head, and it has 

 an outer and an inner wall, enclosing a thin layer of tissue 

 in which lie certain important organs called the shell-glands. 

 Posteriorly to the nuchal grooves the cephalic shield slopes 

 evenly away to the right and left of a median ridge which 

 terminates behind in a spine projecting over the angle of the 



as. 



Fig. 21 



Side view oi Apus cancriformis. The free part of the cephalic shield has been drawn 

 as if transparent, showing the thoracic and abdominal segments beneath. H^ bead ; 

 E^ paired eyes ; c.s^ cephalic shield ; abd^ abdominal segments ; oo^ oostegopod. 



posterior V-shaped excision. The margins of the excision are 

 garnished with a number of similar spines. 



The paired shell-glands lie on either side of the dorsal ridge 

 about half-way between it and the outer margin of the shield. 

 Each gland consists of a tube arranged in a very elongated 

 coil. The anterior ends of the coils converge towards the 

 middle line and touch the nuchal groove ; the 'posterior ends 

 diverge from one another and end some way in front of the 

 posterior margin of the shield. The shell-glands are the 

 functional excretory organs of the adult. 



In front of the nuchal groove there is a complex of organs 

 belonging to the head region. Conspicuous among these is a 

 pair of crescentic compound eyes, the convexities of the cres- 

 cents turned outwards, and their anterior ends closely approxi- 

 mated to one another. The structure of these paired eyes will 

 be described further on, but it should be noticed here that 

 they do not lie at the surface of the shield, but are covered 

 over by a bi-lobed inflation of the chitinous cuticle. This 



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