PHYLLOPODA 329 



cephalic and the thoracic appendages var^'ing in number from 2 pairs 



to 60 : 4 orders with about 4,800 species. 



Key to the orders of Entomostraca : 



Oi Free swimming or parasitic on fish (rarely on other animals). 

 6i Thoracic appendages flattened and leaf-like ; body either elongate and 

 segmented or short and more or less covered with a carapace. 



1. Phyllopoda 



62 Body either elongate and segmented with cylindrical thoracic appendages, 



or greatly modified when the animals are parasites 2. Copepoda 



63 Body short and unsegmented and entirely enclosed in a bivalve cara- 



pace 3. OSTRACODA 



0-2 Body sessile and enclosed in a calcareous shell (barnacles) or parasitic 



on decapods or mollusks 4. Cirripedia 



Okder 1. PHYLLOPODA.* 



Thoracic appendages flat and leaf-like, as the name indicates, being 

 organs of respiration ; body either long and vermiform and composed 

 of numerous segments or short and compact and unsegmented; carapace 

 usually present; parthenogenesis common, the usual eggs being relatively 

 small and thin-shelled and called summer eggs; at the approach of a 

 period of drought or cold males are bom from the parthenogenetie eggs 

 which fertilize the females, and the eggs which these then lay are large 

 and thick-shelled and called resting or winter eggs, and are capable of 

 enduring the winter's cold or the summer's drought, if need be: 2 sub- 

 orders and more than 600 species, most of them being fresh-water animals, 

 living in pools, lakes, and streams containing the minute algae which form 

 their principal food. 



Key to the suborders of Phyllopoda: 



Oi Body elongated and distinctly segmented 1. Branchiopoda 



©2 Body short with indistinct segmentation or without any, and usually with 



a bivalved carapace 2. Cladocera 



Suborder 1. BRANCHIOPODA.f 



Elongated phyllopods with numerous distinctly marked segments, and 

 either with or without a carapace ; the young bom as nauplii : several 

 families and over 100 species, which, with a few exceptions, live in fresh 

 water. 



Key to the families of Branchiopoda here described : 



Oi Carapace absent 1. Branchipodidae 



Oa Carapace present. 



Ih Carapace flattened doj-soventrally and arched 2. Apodidae 



bo Carapace compressed laterally 3. Ltmnaditdae 



* See "Die Siisswasserfaunn Doutsclilands," ITeft 10. 1909. 



t See "Phyllopod Crustacea of North America," by A. S. Packard, 12th Ann. Rep. 

 U. S. Geol. Sur. of the Ter., 1878 (1883), pt. 11, p. 294. 



