ARTHROPODA 



opening on the second antennae. The Entomostracan shell 

 gland, however, is found in the larva, and rudiments of it per- 

 sist in the adult. 



The Leptostraca are exclusively marine ; they appear to 

 be the survivors of what was once, judging from the numerous 

 allied fossil forms, a large order. They possess great tenacity 

 of life, and are able to live in water which is so full of the 

 products of decomposition as to be fatal to most other animals. 

 The order includes three genera, — Nebalia, Paranebalia, and 

 Nebali<ypsis, — :and it is probably cosmopolitan. 



Okdbr 2. THOEACOSTRACA. 



Chaeacteeistics. — Malacostraca in which all the thoracic or the 

 anterior thoracic segments are fused with the head to form « 

 cephalothorax, which is enclosed by a dorsal shield or carapace. 

 The eyes are compound, and, with few escceptions, are borne 

 on moveable statics. 



This order contains four sub-orders, the members of which 

 have attained very different degrees of developement. Whilst 

 some of them in their adult condition have a marked resem- 

 blance to the larvae of the more highly developed groups, others, 

 as for instance the Decapods, are the largest and in some 

 respects the most highly differentiated of the Crustacea. 



The sub-orders which compose the order Thoracostraca 

 are': 



1. CUMACEA. 



2. Stomatopoda. 



3. Schizopoda. 



4. Decapoda. 



Sub-order 1. CUMACEA. 



Characteristics. — The carapace is small, and four or five of the 

 thoracic segments are free. There are two pairs of maxilli- 

 pedes, and six pairs of other thoracic legs ; of the latter the 

 two anterior at least are biramous or Schizopod-like. The 

 abdomen in the female is devoid of appendages, except the sixth 

 segment, which bears a pair of cavdal styles ; the male has a 

 varying number, 2, 3, or 5 pairs of aidominal swimming - 

 feet in addition. 



