AR TIIKoroDA 



I'M 



SUBCLASS II. ARTHROSTRACA 



The Arthrostraca (Gr. apBpov, joint, and oarpaKov, shell) 

 possess the typical twenty segments of the Malacostraca, with 

 a pair of appendages on each except the last. The first tho- 

 racic segment is fused with the head, sometimes the first two ; 

 but all the remaining segments of the body are distinct. The 

 thoracic appendages, except the first pair, are organs of loconio- 



FlG. 153. Asellus aqualicus. A, dorsal aspect; 15, ventral, ant.i, antennule; ant. 2, an- 

 tenna ; t; pouch i-y//,cephalothora.\; E, eve : t.l-l.J, legs ;//./, pl.3, //Apleopods; 

 th.i-th.S, free thoracic segments. (After Gerstaecker, from Parker and Haswell's Text- 

 book.) 



tion and consist of the endopodite only ; the abdominal append- 

 ages are biramous. The eyes in this group are sessile. Two 

 orders are commonly recognized. 



Order i. Isopoda 



The Isopoda (Gr. tVo?, similar, and Trow, foot) are mostly 

 marine ; a few occur in fresh water and on the land. They are 



M 



