257 



i OSTRACODA. 



The species of this order are in the main, during adult life, 

 limicolous forms found in the littoral or bottom ooze or amid the 

 decaying organic matter which accumulates in these regions. The 

 current, the movements of fish and other large aquatic organisms, 

 the action of waves along shore and in shoal regions, all tend to bring 

 these animals into the limnetic fauna. Their centers of distribution 

 are thus in littoral or bottom regions, and in the adult stage they 

 are almost wholly adventitious in the plankton of our waters. In 

 1898 the average number per m. 3 was 191, but in 1897, a more stable 

 year, only 97. 



The seasonal distribution of their occurrences in the plankton 

 indicates a decided predominance in March-October, in which 

 months all but 6 of the 73 records were made. In these months 

 from 23 to 82 per cent, of the collections contained Ostracoda, while 

 in December-February only 8 to 20 per cent. The percentages in 

 April-September are all above 45 per cent., and the numbers per 

 m. 3 are also larger in this period (see Table I.). The tendency 

 toward a vernal increase is apparent in the records of each year in 

 much the form in which it occurs in 1898 (Table I.). The numbers 

 are always small at all seasons, not exceeding 1,600 per m. 3 even 

 in the vernal season. 



The seasonal distribution is such that the greater part of the 

 occurrences and the greater number of individuals appear in the 

 plankton during the warm season, that is, above 50. Thus, in 1898 

 all but 4 of the 24 occurrences and 99.5 per cent, of the indivi- 

 duals appear after the vernal rise passes 50 and before the 

 autumnal decline reaches that point. The Ostracoda are plank- 

 tonts of the warmer season. 



It is significant that the Ostracoda in our plankton collections 

 are largely young or immature individuals. In 1898, 'for example, 

 74 per cent, of individuals observed were not adult, and most of 

 these appeared in April-June. Their occurrence in the plankton 

 can not be traced to the action of flood waters. It thus seems 

 probable that the young Ostracoda may temporarily adopt more 

 of a limnetic habit than the adults. 



No attempt was made to systematically identify the Ostracoda of 

 the plankton catches. The list of species and the notes thereon 



