476 CRUSTACEA MALACOSTRACA. 



Corresponding with this difference in habit there is a well 

 marked sexual dimorphism, the males having the carapace 

 smooth, long posterior antennae, and the differences in the 

 thoracic and abdominal appendages noted above. None of 

 the appendages of the male are modified as copulatory organs. 



The young of the Cumacea are hatched in the form of the 

 adult, except that the eighth thoracic feet are absent, in which 

 as in other features of their development they resemble the 

 Isopoda. 



The arctic seas, up to the edge of the perpetual ice, are especially 

 rich in genera and species of Cumacea and here too and in deep 

 water elsewhere they attain their largest size (35 mm.) They 

 also occur in the antarctic seas, but appear to be scarce in 

 shallow tropical waters. They have been taken from a depth 

 of 2,600 fathoms. The free-swimming shoals are the prey 

 of shoals of herring. Together with species of Mysidae and 

 other relicts of a marine fauna the Cumacea are represented 

 by several species in the salt waters of the Caspian Sea. 



No fossil forms are known. 



The Cumacea are arranged in the following nine families by 

 G. 0. Sars. 



Fam. 1. Cumidae. Carapace comparatively large ; thoracic appen- 

 dages 5-8 without exopodites in both sexes ; 5 pairs of pleopods in 

 male. Telson absent. Cuma M. Edw. ; Iphinoe Sp.B. ; Cyclaspis 

 G.O.S. ; Cyclaspoides Bonnier ; Cumopsis G.O.S. ; Stephanomma G.O.S. 



Fam. 2. Vaunthompsoniidae. General form as in Cumidae, but 

 exopodites are present on thoracic legs 2-4 in the female, 2-5 in the male. 

 Vaunthompsonia Sp.B. 2 spp. known one British (V. cristatd) the other 

 from Kerguelen. Leptocuma G.O.S. ; Heterocuma Miers ; Baihycuma Hansen. 



Fam. 3. Lampropidae. Carapace generally rather small ; exopodites 

 on thoracic legs 4-7 ; pleopods not more than 3 in the male. Telson well 

 developed. Lamprops G.O.S. ; Hemilamprops G.O.S. ; Paralamprops G.O.S. 



Fam. 4. Platyaspidae. Resembles the last, but the carapace very 

 large and flattened, and thoracic appendages 5-8 without exopodites in 

 the male. Platyaspis G.O.S. ; Chalarostylis Norm. 



Fam. 5. Leuconidae. Carapace small ; thoracic appendages 4-7 

 with exopodites ; only two pleopods in the males ; telson absent. Leucon 

 Kroyer ; Eudorella Sp.B. ; Eudorellopsis G.O.S. 



Fam. 6. Diastylidae. Head and thorax tumid, abdomen abruptly 

 narrowed, with freely moving nodular segments and a long distally tapering 

 telson. Thoracic appendages 45 in female, 47 in male with exopodites,. 

 2 pleopods in male, uropods long and slender. Diastylis Say, some 30 

 known spp. (Fig. 292). Diastyloides G.O.S. 1 ; Diastylopsis S. Smith ; Lepto- 

 stylis G.O.S. ; Pachystylis Hansen. Pseudodiastylis and Paradiastylis 

 Caiman. 



