503 



CHAPTER XXIX. 



CR USTA CEA— continued. 



Sub-Class II. — Entomostraca. 



Sub-class II. Entomostraca (Gnathopoda, Woodward). — The 

 division of the Entomostracous Crustaceans includes a large number 

 of comparatively simple types, in which the limbs and segments are 

 usually indefinite in number, the former either fewer or more than 

 fourteen ; and the character of the appendages is very varied. The 

 limbs are principally developed in the cephalic region, and their 

 bases generally act as jaws. The characteristic larval form is that of 

 a " nauplius." The orders of the Eurypterida and Xiphosura (with 

 the probable addition of the Trilobita), here placed among the 

 Entomostraca, are grouped together by Professor Claus in a special 

 section, which he terms Gigantostraca, and which he regards as 

 probably related to the Arachnida. 



The Entomostraca are divided into three great divisions, or 

 " legions," the Lophyropoda, Branchiopoda, and Merostomata. 



Division A. Lophyropoda. 



The members of this division possess few branchiae, and these are 

 attached to the appendages of the mouth. The feet are few in num- 

 ber, and mainly subserve locomotion ; the carapace is in the form 

 either of a shield protecting the cephalothorax, or of a bivalve shell 

 enclosing the entire body. The mouth is mostly not suctorial, but 

 is furnished with organs of mastication. 



This division comprises the two orders Ostracoda and Copepoda. 



Order I. Ostracoda. — Small Crustaceans having the entire body 

 enclosed in a shell or carapace, which is composed of two valves united 

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

 by an adductor muscle, the insertion of which in the interior of each 



