196 



Arthropoda. Class 1. Crustacea. 



but special vessels are wanting. A well-developed shell-gland is 

 present. Some Daphnids differ from the usual type now described, 

 in that the carapace is wanting, or is only feebly developed, that the 

 body is elongate, and that the thoracic-limbs are aberrant in form. 



The Daphnids are small (at most a few m/m. long), transparent 

 animals, which are mostly fresh- water, though a few are marine; 

 they move through the water by jumps (Water-fleas). During the 

 summer, usually only females are found, producing partheno- 

 genetically large, thin-shelled " summer eggs," which are 

 " brooded " in the cavity between the trunk and the carapace ; the 

 young ones leave the brood-pouch almost in the condition of the 

 parent; in autumn males appear also. The fertilised ova, "winter 

 eggs" ("resting eggs"), which are thicker-shelled than the summer 



ones, usually pass the winter 

 inapeculiarcase {Ephippium], 

 formed by the thickened 

 cuticle of the whole or part 

 of the carapace, and thrown 

 off by the female together 

 with the eggs. The winter 

 eggs develop in the spring ; 

 in some forms the young ones 

 hatch out of the winter 

 as nauplii. 



Order 3. Xiphosura. 



In the living Xiphosura, 

 which comprise only a single 

 genus, Limulus (the King- 

 crab), the body is divisible 

 into two unsegmented parts, 

 the cep halo-thorax and 

 the abdomen, which are 

 movably articulated ; each of 

 these regions is composed 

 of a number of fused segments. 

 The cephalo-thorax is strongly 

 arched, the sides are thin and 

 continued down to form a 

 shield-shaped structure, which 

 covers in the ambulatory 

 appendages. A carapace is 

 wanting. Upon the dorsal 

 surface there is a pair of large 



Fig. 157. Limulus polypJiemusi $ , from 

 below : reduced. 1—6 ambulatory appendages, 

 opercvilum of the gill-bearing- limbs, the edges 

 of which are seen one behind the other. 



