PHYLUM ARTHROPODA 



303 



verted eye found in the phylum Arthropoda, and is another distinguish- 

 ing feature of the class Crustacea. 



Most daphnias are females which reproduce parthenogenetically. 

 Paired ovaries lie beside the midgut. Eggs are laid through ducts that 

 open dorsally into a brood pouch, an enlarged cavity between the back 

 of the body and the carapace. The eggs remain here until they develop 

 into small daphnias resembling their parents. When the environment 

 becomes unfavorable (too cold, no food, etc.) some of the young mature 

 as males while the females produce "resthig eggs." These are fertilized 

 and shed to the bottom where they may last for years without hatching. 

 The same females produce both parthenogenetic and resting eggs, de- 

 pending upon whether the environment is favorable or unfavorable. 



138. Other Crustaceans 



Small crustaceans are usually considered to be the more primitive 

 Crustacea. Of these a natural group is formed by the orders Anostraca 

 (brine shrimps and fairy shrimps), Notostraca, Conchostraca and Clado- 

 cera (water-fleas) (Fig. 16.12), in which the trunk limbs are biramous, 

 flattened, and used for both respiration and feednig. These orders form 

 the subclass Branchiopoda. They are mostly fresh-water organisms, and 

 are especially abundant in temporary ponds. 



Other small crustaceans include the orders Ostracoda, Copepoda 

 and Cirripedia (Fig. 16.18). The first two are common in both fresh 

 and salt water. The last are the barnacles, found only in the seas. Cope- 

 pods are the most abundant of all crustaceans, forming dominant or- 

 ganisms of salt and fresh-water plankton. The evolutionary relations of 

 these groups to each other, to the Branchiopoda, and to the Malacos- 

 traca are somewhat obscure. 





^ >^N v'*"'^ V ^^ 







</' 



Anostrstca. 



^^^^M-" 



"CoTicbostra.C3u 





Motostra-c 



Figure 1 6.1 2. Other members of the subclass Branchiopoda. (After Borradaile et al.) 



