289 



gelatinous coenoecia are spherical, ellipsoidal, or often somewhat 

 flattened. The longest diameter of these floating masses often 

 exceeds 30 cm. 



Plumatella repens L. This is by far the most abundant bryozoan 

 in our locality, being found everywhere on submerged vegetation 

 in the backwaters. It often develops with surprising rapidity on 

 the submerged stems of plants, where, as in 1896, summer floods 

 reinvade the vegetation-covered margins of reservoir backw r aters. 

 It is represented in the plankton by its floating statoblasts. Their 

 seasonal distribution shows some correlations with temperature, 

 hydrographic conditions, and the seasonal cycle of the parent 

 organisms. During the period of minimum temperatures (Decem- 

 ber-February, inclusive) they are relatively rare in the plankton, 

 appearing in 30, 8, and 20 per cent., respectively, of the plankton 

 catches. They are rare in high- as well as low-water conditions, as, 

 for example, in the floods of 1895-96 and 1898, when they appear 

 in but one of 15 collections. With the rise of temperature in March 

 they occur more frequently, as, for instance, in 1898 (Table I.), and 

 continue during the run-off of the spring flood. The occurrences 

 rise in March-May to 60, 46, and 50 per cent, of our total collections 

 in these months, and the numbers also are larger. For example, in 

 1898, 81 per cent, of the total individuals for the year were found 

 in these months. The discharge from impounding backwaters, the 

 principal breeding grounds of the parent organisms, doubtless tends 

 to increase the numbers of statoblasts in channel plankton during 

 this season. During the remainder of the year, June-November, 

 the percentage of occurrences again falls to 30, 50, 24, 32, 18, and 

 44 per cent., respectively. The 50 per cent, in July is due to the 

 summer flood of 1896. If this year is omitted the record falls to 

 33 per cent. The large percentage for November is probably due to 

 the predominantly higher levels of this month, to the invasion of 

 lake margins seeded with statoblasts, and to the increased activity 

 in the fishing industry, which tends to disturb the summer's growth 

 of vegetation in tributary backwaters. The relations to the seasonal 

 cycle of the species are patent. The summer months, June- 

 September, are the season of growth and spread of the parent 

 organisms and of the formation of statoblasts, especially as receding 

 levels expose the water margins. Hydrographic or other disturb- 

 ances tend to increase the number of statoblasts in the plankton 



