480 



CRUSTACEA MALACOSTRACA. 



lamellar legs, whose inner rami serve as branchiae, on the short- 

 ringed, often reduced abdomen. 



The body is flat and covered by a firm, usually calcareous 

 integument, and presents in its 

 structure a greafc agreement with 

 that of the Amphipoda. The ab- 

 domen is, however, usually much 

 shortened and the segments are often 

 fused with one another forming a 

 large caudal shield. The endopodites 

 and sometimes also the exopodites of 

 the abdominal legs are modified as 

 branchial lamellae. 



Isopods are in great part littoral 

 Crustacea though some species (re- 

 markable for their large size, and 

 development of spines, Beddard) in- 

 habit the bottom of the sea at great 

 depths, others (e.g., Asellus) are in- 

 habitants- of fresh waters, and one 

 group, the Oniscidae, is terrestrial. 

 Besides these free-living forms many 

 are parasitic on the bodies of fishes 

 and Crustacea and in their structure 

 and mode of life depart in varying 

 degrees from the type of the free 

 living forms, which however always 

 recurs in the larval stages. 



The anterior antennae are, with a 

 few exceptions, shorter than the pos- 

 terior and external antennae ; in rare cases (Oniscidae) they 

 become so much reduced that they are hidden beneath the 

 cephalic shield. 



As in the Amphipoda delicate plumose setae and olfactory 



FIG. 295. -Asellus aquaticus (after 

 G. O. Sars). Female with brood 

 pouch, seen from the ventral side. 



familiae nonnullae propinquae, Vid. Selsk. Skrift. Kjobenhaven, 1890. 

 J. Nusbaum, Materialien zur Embryogenie u. Histogenie d. Isopoden, 

 Abh. Akad. Krakau, 1893 (Polish). J. H. Stoller, On the organs of respira- 

 tion of the Oniscidae, Bibl. Zool. Heft, xxv, 1899. G. O. Sars, Hist. Nat. 

 d. Crustaces d'eau douce de Norvege, Christiania, 1867, and Crustacea 

 of Norway, vol. ii., Isopoda, 1896-99. G. Smith, Fauna u. Flora des 

 Golfes von Neapel, Monog. 29, Rhizocephala, 1906, etc. 



