212 INVERTEBRATE ZOOLOGY 



heart allows the blood to enter, and contraction of the heart 

 forces the blood through a single anterior opening. There are 

 no specialized organs for respiration. That portion of the body 

 within the shell is divisible into two regions, one, the body proper 

 which is not sharply segmented but bears six pairs of foliaceous 

 appendages, and the other, an unsegmented postabdomen. 

 The trunk appendages function chiefly in creating water currents 

 through the space within the shell. 



Most Cladocera react positively to weak light and negatively 

 to strong light, but these reactions may be reversed by other 

 stimuli acting as controlling factors in their behaviour. Thus, 

 in cold water, Cladocera may respond positively to a light 

 stimulus which would repel the same individuals living at a higher 

 temperature. 



Reproduction is largely parthenogenetic. The eggs are stored, 

 frequently in considerable numbers, in a brood case formed 

 within the dorsal portion of the carapace. Here they undergo 

 full development without ever having a free larval stage. Under 

 ordinary circumstances, all of the parthenogenetic eggs produce 

 females. When unfavorable conditions arise, however, not all 

 of the parthenogenetically developed individuals are females, but 

 some males are hatched from parthenogenetic eggs. In the 

 sexual cycle which follows, each female produces only one or 

 two large thick-shelled eggs. These are true sexual eggs which 

 require fertilization before development is initiated. The 

 fertilized eggs pass into a brood sac the walls or portions of the 

 walls of which become modified as an enclosing envelope (ephip- 

 pium) within which the resting eggs lie until the return of 

 conditions favorable for their development. 



The Cladocera, along with some other crustaceans, have 

 great value in that they are important as food for many aquatic 

 animals, especially fish, while they in turn utilize the smaller 

 algae which are abundant under conditions in which they are 

 found. Daphnia and Bosmina are characteristic shelled forms, 

 while Leptodora and Polyphemus have greatly reduced shells. 

 Most cladocerans live in fresh water. 



Subclass CoPEPODA 



Copepods inhabit both fresh and salt water. Some forms 

 have become so highly modified through adaptation to the 

 parasitic habit that they are recognizable only with difficulty as 



