380 E. A. ANDREWS. 



The food of the case-inhabiting Folliculina being bacteria and 

 some larger forms of plankton, the disappearance of Folliculina 

 may well be associated with changes in food supply, in turn 

 brought about in connection with such changes as those of tem- 

 perature and salinity. 



The motile forms take no food and may be enabled to settle 

 and to continue migration and multiplication only when feeding 

 conditions allow the sessile form to accumulate enough energy. 



SUMMARY. 



1. The vast swarms of swimming protozoans of the genus 

 Folliculina that were found to settle down over the aquatic plants 

 along the shores of side branches of the Chesapeake Bay in 1912 

 and 1913, came in even greater numbers in 1914, and it is there- 

 fore probable that this immigration and colonization is a regular 

 annual phenomenon. 



2. The incursions of swimming Folliculina do not take place 

 as soon as the plants have grown enough to supply places for 

 attachment, and the departure or disappearance of the living 

 Folliculinas antedates the cessation of growth and final dying down 

 of the plants upon which they settle. 



3. As far as evidence is available the numbers that crowd the 

 leaves arise more from immigration from without the area than 

 from division of animals that have already settled in the area. 



4. The times of appearance and disappearance differ in suc- 

 cessive years. 



5. It is suggested that conditions of food possibilities are 

 determining factors in these inroads into the brackish fauna. 



6. The great number of free swimming forms makes them, for 

 the time being, an important factor in the plankton. 



7. The crowding of the dwellings or cases on the leaves all along 

 the shores is a considerable element in the transformation of 

 matter which, arising from decay of organic materials, is trans- 

 formed into bacteria and other plankton organisms, which in 

 turn are eaten by Folliculina and enable them to secrete resisting 

 tubes and sacs which finally settle to the bottom of the river. 



