186 CRUSTACEA. PHYLLOPODA. 



form the body is unsegmented and possesses three pairs 

 of appendages representing the two pairs of antennae and 

 the mandibles. In the Zocea stage some of the thoracic 

 appendages are also present, and the abdomen is seg- 

 mented but possesses no appendages. 



The Crustacea are divided into two large groups, 

 (1) the Eritomostraca, (2) the Malacostraca. 



ENTOMOSTRACA. 



The forms included in this group are mostly of small 

 size and of comparatively simple organisation. The num- 

 ber of segments in the body is variable, and also the 

 number and form of the appendages. In the development 

 there is nearly always a Nauplius stage. There are four 

 orders, (1) Phyllopoda, (2) Ostracoda, (3) Copepoda, 

 (4) Cirripedia. The Copepods are not definitely known 

 as fossils. 



ORDER. PHYLLOPODA. 



The Phyllopoda includes the water-fleas (Daphnia) 

 and other forms. The body is segmented and the greater 

 part of it is covered by a shield-like carapace or by a 

 bivalved shell. The number of segments in the abdomen 

 and thorax varies very widely. On the head there are 

 generally two pairs of antennae, one of mandibles, and one 

 or two of maxillae. The thorax bears four or more pairs 

 of swimming-feet, which are leaf-like. The abdomen is 

 frequently without appendages ; it often ends in a caudal 

 fork. Two large eyes are present and often also a small 

 unpaired eye. The Phyllopods live mainly in fresh or 

 brackish water. Only a very few genera are found fossil ; 

 the earliest occur in the Devonian. 



