SECT. XI 



PHYLUM ARTHROPODA 



515 



alike that, save in minor details, the same description will apply 

 to any species of either genus. 



External Characters. The animal (Fig. 427) 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. 428). 

 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. 428), 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 segmenta- 

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

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

 slightly overlaps its successor, and is connected with it by a narrow 



L L 2 



Fio. 427. Apus cancriformis, dorsal aspect, abd 

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

 paired eye ; e. median eye ; sh. gl. shell-gland ; 

 th. f. 1, endites of first thoracic foot. (From Bronn's 

 TMerreieh.) 



