210 INVERTEBRATE ZOOLOGY 



doxides, and Homalonotus are examples of the 200 or more 

 described genera. 



Subclass Phyllopoda 



Of the present-day Crustacea, the Phyllopoda are the most 

 primitive. The name intimates one of the outstanding charac- 

 ters of the group, the presence of leaflike appendages, of which 

 there are ten or more pairs located on the trunk. Here belong 

 the fairy shrimps (Branchiopoda) and that host of minute, 

 shelled crustaceans known as the water fleas (Cladocera). Fresh- 

 water, marine, and brackish-water forms are included within the 

 confines of this group the members of which have little more than 

 the leaf like appendages as a common characteristic. In the two 

 orders, Branchiopoda and Cladocera, members of the former have 

 numerous segments, very distinctly marked, while members of 

 the latter order have few somites and these are frequently not 

 clearly defined. Even within each order, body form and struc- 

 ture are subject to great variation. There is usually a pair of 

 conspicuous eyes and frequently in addition there is a small 

 median eye. In some instances, the paired eyes, distinct in the 

 young, fuse to form a single eye, but even then two optic nerves 

 are retained, so the double nature of the eye is still observable. 

 The sexes are distinct, though males are much less numerous than 

 females. 



Members of the two orders Branchiopoda and Cladocera are 

 separable on the basis of the number of trunk appendages. In 

 the former, there are ten or more pairs of trunk appendages, while 

 in the Cladocera the appendages of this region do not exceed six 

 pairs. 



I. Order Branchiopoda 



With the exception of one genus, Artemia, the Branchiopoda 

 are exclusively fresh-water phyllopods which occur in all parts of 

 the world. Eubranchipus (the fairy shrimp. Fig. 97), Apus, 

 and Estheria represent three distinctly different types of struc- 

 ture within this order. In habits, all of these are peculiar in that 

 they are restricted to small pools and especially the temporary 

 pools which are formed by spring rains and disappear during the 

 summer. In these temporary pools, they appear in very great 

 numbers in early spring, then after a few days or weeks they dis- 

 appear entirely. An examination of the mud after the pool has 

 dried up reveals large numbers of the eggs which are capable of 



