341 



CHAPTER XVIII. 



CRUSTACEA (Continued). 



SUB-CLASS ENTOMOSTRACA. 



THE Entomostracous Crustaceans are defined by Professor 

 Itupert Jones as follows : " Animal aquatic, covered with a 

 shell, or carapace, of a horny consistency, formed of one or more 

 pieces, in some genera resembling a cuirass or buckler, and in 

 others a bivalve shell, which completely or in great part envelops 

 the body and limbs of the animal ; in other genera the animal 

 is invested with a multivalve carapace, like jointed plate- 

 armour ; the branchice are attached either to the feet or to the 

 organs of mastication ; the limbs are jointed, and more or less 

 sctiferous. The animals, for the most part, undergo a regular 

 moulting or change of shell, as they grow ; in some cases this 

 amounts to a species of transformation" 



The orders commonly included in the sub-class Entomos- 

 traca are the Ostracoda, Copepoda, Cladocera, Phyllopoda, Tri- 

 lobita, and Merostomata (comprising the sub-orders Xiphosura 

 and Eurypterida). Of these, the Copepoda and Cladocera 

 may be left out of consideration, as they are not certainly 

 known to occur in the fossil condition. 



ORDER OSTRACODA. 



Minute Crustaceans having the entire body enclosed in a 

 sJwll or carapace, which is composed of two valves united along 

 the back by a membrane. The valves are capable of being 



