ARTHROPODA 281 
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,—WNebalia, Paranebalia, and 
Nebaliopsis,—and it is probably cosmopolitan. 
OrpEeR 2. THORACOSTRACA. 
CHARACTERISTICS.—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 exceptions, are borne 
on moveable stalks. 
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 
thoracie segments are free. There are two pairs of maxilli- 
pedes, and six pairs of other thoracic legs; of the latter the 
two antervor 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 caudal styles ; the male has a 
varying number, 2,3, or 5 pairs of abdominal swimming- 
Jeet in addition. 
