1 64 SYSTEMATIC ZOOLOGY 



as gills ; and in the female some of them are provided with 

 thin chitinous plates on the inner side, which by overlapping 

 form a brood pouch for the developing young, very much as in 

 the preceding order. Of the six pairs of biramous abdominal 

 appendages usually present, the three anterior pairs are used 

 for swimming, and the three posterior are used in jumping. 

 In some genera the abdomen is very much reduced and ap- 

 pendages are wanting, as in the bizarre Caprella (Fig. 156), 

 which is commonly found crawling amongst hydroids. 



SUBCLASS III. THORACOSTRACA 



The Thoracostraca(Gr. Ocopa^, breastplate, and oarpaicov, shell) 

 are readily distinguished from the Arthrostraca by two striking 

 differences. We noted in the Arthrostraca that only the first 

 thoracic segment was fused with the head and that the remain- 

 ing seven were distinct. In the Thoracostraca, all or nearly all 

 of the segments of the thorax fuse with the head, forming a more 

 or less complete cephalothorax ; and this entire region is then 

 covered by a single piece of shell, the carapace, which also 

 extends down over the sides of the body and covers the gills, 

 when these are attached to the thoracic appendages, and thus 

 forms a gill chamber. In the abdomen, the segments are dis- 

 tinct. The second point of difference lies in the eyes, which 

 in the Arthrostraca are sessile, and in the Thoracostraca are 

 situated at the free ends of rather long stalks. This group is 

 divided into four orders. 



Order 1. Cumacea 



The Cumacea (Gr. kv/jlci, anything swollen, wavelike) pre- 

 sent the one exception to the statement just made, that the eyes 

 of the Thoracostraca are stalked ; this is the only order in which 

 they are sessile. The carapace covers only the first three or 

 four thoracic segments ; the remaining four or five are dis- 

 tinct. The Cumacea are marine and form a small order. A 

 common genus is Diastylis (Fig. 157), about a centimeter in 

 length. 



