INTRODUCTION 13 



in open sunny niarshos and ditrlu's is prohahiN' due to tlu' laiKc amount 

 of other organic life in such places. 



As to parasitism, of which much remains unknown, Petersen finds 

 that fish are little attacked in natural conditions in Denmark and then only 

 as a result of pre\ious injury. He seems to think that frog eggs may be 

 attacked in the li\ing condition. We have often seen eggs of frogs and 

 salamanders overgrown witli water molds, Init we ha\e no proof that 

 these plants were the actual cause of the death of the eggs. Petersen 

 also announces that Leptolegnia caudata is the fungus which attacks 

 and kills great numbers of the crustacean Leptodora in his country. 

 We have not found it a parasite here. 



In order to throw light if possible on the seasonal distribution of 

 the different species we have made up the table that follows this page. 



Between February 15, 1912, and December 15, 1913, the number 

 of collections made from all sources around and in Chapel Hill was 

 593. A record of the species found in each collection was kept and from 

 these records and the number and date of the collections the table was 

 drawn up. It shows the per cent of all collections in which each species 

 occurred for each month, for spring, summer, fall and winter; for the 

 six warmest months (April-September inclusive) and the six coolest 

 months, and finally the per cent of the whole number of collections in 

 which the species was found compared to the total number of collections 

 made in the entire period. For example, Achlya apiculata was found 89 

 times in 593 collections or in 15'^oofall collections. It was found 33 times 

 in 138 collections made in March, or in 23.9% of the March collections. 

 The number of collections made in each month varied considerably, for 

 example, 24 collections were made in July and 138 in March. While 

 this does not interfere with accuracy of the percentages, it does give 

 opportunity for a larger variation of percentages in months with fewest 

 collections, because of the various accidental factors involved in collecting. 

 That is, the larger the number of collections the less the percentage of 

 error in the figure showing the occurrence of a certain species at a certain 

 place for a certain time. As would be expected, the number of collections 

 in warm months was less than in the cool months, due to the absence 

 of members of the staff on holidays. There were 22)J collections made in 

 the six warm months and 356 in the six cool months. Since 1913 we 

 have continued steadily at work on this group, but ha\e kept no exact 

 data as to the number of collections, which will easily run into several 

 thousands. 



It will be noted that Achlya flagellata and A. proliferoides have been 

 run together in the table. This is due to the fact that the two species 

 were not distinguished for some time after the records began and went in 



