1 66 



SYSTEMATIC ZOOLOGY 



appendages are mouth parts, and the more posterior of these 

 are very large and strong ; they consist of a single row of joints, 

 while the remaining three pairs of thoracic appendages are 

 biramous. The abdomen is strongly developed, and the six pairs 

 of appendages are large, biramous, swimming organs ; to the 

 first five pairs of these gills are attached. 



Order 3. Schizopoda 



The Schizopoda (Gr. cr^t'^a, cleft, and ttovs, foot) are small 

 marine Thoracostraca, rarely more than half a centimeter in 

 length and often less. They are transparent, shrimplike, and 



derive their name from the eight 

 pairs of thoracic appendages, which 

 are all very much alike and bira- 

 / mous, hence "split-footed." Gills 



are sometimes present on the tho- 



FlG. 159. Mysis oculata, enlarged, end, endopodite ; ex, exopodite ; ot, otoi rood 



pouch. (After Gerstaecker, from Parker and Haswell's Text-book.) 



racic appendages. The carapace covers practically the whole 

 thorax, though one or two of the posterior segments may remain 

 distinct. The sixth abdominal segment has a pair of large ap- 

 pendages, which together with the seventh segment, or telson, 

 form the flapper or tail fin, and on these appendages is found in 

 some species the curiously placed ear or auditory vesicle. A com- 

 mon representation of this order is the genus Mysis (Fig. 159). 



Order 4. Decapoda 



The Decapoda (Gr. Be'ica, ten, and 7row, foot) include the 

 largest and most highly developed Crustacea. The order 

 receives its name from the five posterior pairs of appendages on 

 the thorax, which are always well developed and emploved for 

 the most part for walking. In this group there is a complete 



