120 



CRUSTACEA PERACARIDA 



CHAP. 



related genera compose this family, of which J/yst's, Boreomysis, 



and Siriella may be mentioned. My sis 

 oculata, var. relic ta, is a freshwater 

 form from the lakes of northern and 

 central Europe. 



Order II. Cumacea. 1 



Ab,6 



The Cumacea are a group of small 

 marine animals rarely attaining an 

 inch in length, which agree with the 

 Mysidacea in the characters noted 

 above as diagnostic of the Division 

 Peracarida ; they possess, however, 

 in addition a number of peculiar 

 properties, and Sars believes them 

 to be of a primitive nature showing 

 relationship to Nelialia, and possibly 

 to an ancestral Zoaea - like form. 

 They follow a habit similar to that 

 of the Mysidacea, being caught either 

 in the surface -plankton or in great 

 depths, many of the deep-sea forms 

 being blind. They are, however, not 

 true plankton forms, and they appear 

 to attain a greater development both 

 in point of variety and size in the 

 seas of the northern hemisphere. The 

 thoracic limbs may be biramous, but 

 there is a tendency among many of 

 the genera to lose the exopodites of 

 some of the thoracic legs, an exopodite 

 never being present on the last few 

 thoracic limbs of the female and on 



FIG. 80. Dorsal view of male 



Diastylisstygia,Kl2. A, 2nd the last in the male. In the Cumidae 



the four posterior pairs in both sexes 

 have no exopodites. The first three 

 thoracic appendages following the maxillae are distinguished as 

 inaxillipedes ; they are nniramous, and the first pair carries an 



1 8;ir.s. "Crustacea of Norway," in., 1900. 



