362 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 112 



rated in time to provide the details needed for a satisfactory 

 explanation. 



Development: As in almost all amphipods, the eggs and early 

 juveniles are carried loosely in the marsupium between the ventral 

 wall of the pereon and the oostegites. Counts made of the number of 

 eggs ranged from 20 to 60, with an average number of about 35. 

 Behning (1939) found up to 200 eggs per female in the much larger 

 (8.5-17 mm.) P. japonica. 



When the young leave the marsupium there is almost no difference 

 between the sexes. The young are miniature editions of the adults, 

 except that there are fewer setae on the pereopods, and the flagella 

 of antennae 1 and 2 are unisegmental in both sexes. Of course the 

 oostegites are entirely absent from the female. 



Even at this early stage it is possible to distinguish males from 

 females in almost all cases by the shapes of the antennal flagellae, 

 especially those of antennae 1. In the early juvenile male the 

 flagellum of antenna 1 is longer and broader; the portion distal to the 

 sensory filaments, which in the female is shorter and sharply pointed, 

 tapers more gradually and is half the length of the whole flagellum. 

 The flagellum of antenna 2 is also longer and heavier in the male. 



As development proceeds, the male flagellum becomes segmented. 

 In males about to molt, the segmentation which the flagellum will 

 have during the following instar can be seen through the cuticle. 

 Consequently it has been possible to estimate the number of instars 

 from the last stage characterized by a unisegmental flagellum to the 

 adult male stage. 



The instars may be defined as follows : 



1. Flagellum of antenna 1 short, unisegmental. 



2. Flagellum slightly shorter than height of head, 8-10 segmented. 



3. Flagellum about as long as pereonites 1-5 combined, 11-12 



segmented. 



4. Adult male, flagellum about as long as pereon, 13-15 segmented. 



The additional segments are added from the distal end of the long 

 first flagellar segment. In the male about to molt, it can be seen that 

 two or three segments are formed here. The segments distal to the 

 first one apparently do not divide further after being formed. 



This method of forming additional segments differs from that of the 

 gammarids Pontoporeia affinis Lindstrom (Segerstrale, 1937) and 

 Oammarus chevreuxi Sexton (Sexton, 1924). In Pontoporeia males, 

 new segments are formed, as in Parathemisto, by division of the 

 proximal segment. At first this is the only method, but later the 

 remaining segments begin to divide into two, after which the} 7- do not 

 divide again. In the female the distal segments do not divide, and 

 the process is essentially similar to that in Parathemisto. In 



