PHYLUM ARTHROPODA 



527 



th.f.1 



in 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. 420) is from 20 to 

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

 surface covered by a thin chitinous shell or carapace, beyond the 

 posterior edge of which the hinder part of the body (abd.) projects 

 as a nearly cylindrical structure distinctly divided into segments. 

 The last or anal segment 

 bears a pair of long 

 processes, the caudal 

 styles (a. /.) between 

 which, in Lepidurus, is 

 a flat scale-like post- 

 anal plate (Fig. 421). 

 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 

 a semi-transparent oval 

 area, the dorsal organ 

 (d. o.). Passing trans- 

 versely across the cara- 

 pace, a short distance 

 behind the dorsal organ, 

 is a shallow furrow, the 

 cervical fold, immedi- 

 ately 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. 421), 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 segmentation, and is distinguished as the head. The 

 segments have the form of chitinous rings, often produced into 



PH.;. 420. Apus cancriformis, dorsal aspect, abd. 

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

 paired eye ; e. median eye ; sh. gl. shell -gland ; th.f. 1, 

 endites of first thoracic foot. (From Bronn's Thierreich.) 





