206 



ARTHROPODA 



[CH. XII 



but the second antennae are very large and forked and project 

 from the shell, and by their lashing movement carry the animal 



through the water (Figs. 78 and 79). 

 The carapace is to a certain extent 

 transparent, and through it the beat- 

 ing of the heart, the circulation of 

 the blood and the movements of the 

 thoracic leaf-like appendages may be 

 made out. Within the substance of 

 each valve of the carapace a coiled 

 glandular tube may be detected; this 

 is the shell-gland or typical ex- 

 cretory organ of the Entomostraca 

 which opens to the exterior in the 

 region of the second maxilla. The 

 male (Fig. 78) is usually smaller 

 than the female (Fig. 79), and is cer- 

 tainly very much rarer. The females 

 lay two kinds of eggs, (i) unfertilised 

 eggs, which develop in the space inter- 

 vening between the dorsal side of the 

 body and the shell which acts as a 

 brood-pouch, and (ii) fertilised eggs, 

 which are larger and become sur- 

 rounded by a special modification of 

 the brood-pouch called the ephippium. 

 The nature of the eggs produced is 

 regulated by favourable or unfavourable 

 conditions of life. At a suitable tempe- 

 rature and with a sufficiency of food 

 and water, the unfertilised eggs are 

 produced in large numbers at short 

 intervals. Periods of drought or the 

 cold of winter bring about the forma- 

 tion of eggs which are fertilised and 

 enclosed in the ephippium. Sheltered 

 by this case the eggs are enabled to 

 withstand freezing or desiccation and 

 with a return of suitable conditions 

 a young Daphnia hatches out from 

 each egg to continue the cycle of life. 



FIG. 77. Dorsal view of female, 

 Branchipus sp. found in a 

 pond in Sussex x about 10. 



1. Antennae. 2. Head. 3. Eyes. 

 414. The eleven ' ' thoracic " 

 limbs. 15. The caudal forks. 

 16. The fifth "abdominal" 

 segment. 



