4 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvii. 



VI. Oniscoidea. 



1. ()niscid;e. 



2. Arinadillidiiche. 



3. Ligiitl;e. 



4. Trichoniscidse. 



5. Tylida;. 



6. Helleriidie. 



VII. Epicakidea or Boi'ykoidea.^ 



1. Bopyridie. 



2. Entoniscidfc. 



3. Dajidfc. 



4. Cryptoniscidse. 



Further .subdivision into g-enera and species is not desirable, although 

 the genera and species will be constantly referred to in the following 

 pages. 



2. EXTERNAL ANATOMY. 



A. GENERAL FORM. 



The body is generally" flattened or compressed dorso-ventrally, dif- 

 fering in this respect from the Amphipoda, their nearest relatives, 

 which have the body flattened or compressed laterally. The outline 

 of the bodj' varies considerably. Manj'^ forms are narrow and elon- 

 gate, the extreme of this tj^pe being found among the Anthurida^.;* 

 others are broadly oval, almost circular in outline (the Serolid?e). 

 Bilateral symmetry prevails in the group as a whole. The adult 

 females in the parasitic Isopoda deviate from the S3'mmetr3" of the 

 larval condition, owing to the parasitic mode of life, the parasitism 

 being also accompanied in some of the Epicaridea by a degradation in 

 structure of such an extent that the characters of the group are 

 entirely lost, their relationship being known only through a study of 

 the early larval stages and through the males which retain the charac- 

 ters of the group. Some of the Cymothoidse, also parasites, are as 

 symmetrical in the adult condition. 



Sexual dimorphism occurs only in the parasitic tribe Epicaridea. 

 The males in this superfamily are four or live times smaller than the 

 females and are always found permanently attached to the body of 

 the female. 



In both sexes the body is segmented, the number of segments into 

 which the thorax and abdomen is divided differing in the various sub- 

 divisions of the group. Fusion of the first and second segments with 

 the head occurs sometimes, and often some or all of the abdominal 

 segments are coalesced. The Dajidge have the body very imperfectly 



« The second name given above for the tribe or superfamily was suggested by 

 Dr. Theodore Gill in order that the nomenclature might be more uniform. 



& In Hyssura vermiformis Haswell, a member of this family, all the segments of 

 the body are extremely narrow and elongated, giving the body an exceedingly long 

 appearance. 



