SECT. XI 



PHYLUM ARTHROPODA 



485 



thf-1 



distribution and by no means common. They are so much alike 

 that, save in minor details, the same description will apply to any 

 species of either genus. 



External Characters. The animal (Fig. 386) is from 20 to 

 30mm. in length, and has the anterior two-thirds of the dorsal surface 

 covered by a thin chitinous shell or carapace, beyond the 

 edge of which the hinder part of the body (dbd.) projects 

 nearly cylindrical structure distinctly divided into segments. 

 last or anal segment * 



bears a pair of long e 



processes, the caudal 

 styles (a. /.) between 

 which, in Lepidurus, is 

 a flat scale-like post- 

 anal plate (Fig. 387). 

 On the dorsal surface of 

 the carapace, near its 

 anterior border, are the 

 paired eyes (E\ closely 

 approximated in front, 

 diverging posteriorly. 

 Immediately in front 

 of them is a small 

 black median eye (e.), 

 and between their di- 

 verging posterior ends 

 is a semi-transparent 

 oval area, the dorsal 

 organ (d. o.). Passing 

 transversely across the 

 carapace, a short dis- 

 tance behind the dor- 

 sal organ, is a shallow 

 furrow, the cervical fold, 

 immediately posterior 

 to which a pair of 

 coiled tubes (sh.gl.) are 



seen, one on each side of the carapace : these are the shell-glands 

 or excretory organs. 



The carapace is attached only as far back as the cervical fold : 

 behind that level it is free, and. when lifted up or cut away 

 (Fig. 387), shows the greater part of the body of the animal, divided 

 into segments like the posterior portion. From the cervical groove 

 backwards about twenty-eight or thirty segments can be counted : 

 the region in front of the cervical groove shows no sign of segmen- 

 tation, and is distinguished as the head. The segments have the 

 form of chitinous rings, often produced into small spines : each 



FIG. 386. Apus cancriformis, dorsal aspect. aM. 

 abdomen ; a. /. caudal styles ; d. o. dorsal organ ; E. 

 paired eye; e. median eye; sh. gl. shell -gland; th.f.l, 

 endites of first thoracic foot. (From Broun's Thier-reich.) 



