INTRODUCTION 



7 



those that (Fig. i) pass through the meshes of the finest silk gauze, 

 Swiss bolting cloth No. 25,* having meshes that measure 0.04 to 

 0.05 mm. square. The nannoplankton is composed chiefly of 

 flagellates and algas ; although bacteria are constantly present they 

 apparently form but a minor con- 

 stituent in bulk and weight. The 

 number and variety of these or- 

 ganisms is truly astonishing even 

 in the clear waters of Alpine 

 lakes where according to Ruttner 

 they stand to the organisms of 

 the net plankton numerically in 

 the ratio of 160 : 3 and at least 

 two-thirds of them are still un- 

 described and difficult to include 

 in known genera. The maximum 

 number of nannoplanktonts thus 

 far recorded is from Lake Men- 

 dota, Wis., where Cyclotella has 

 been found to the number of over 

 30,000,000 per liter of water. 



Ruttner also calcvdates the vol- 

 ume of the nannoplankton in the 

 Lunzer lakes as three times that 

 of net plankton. According to 

 Birge and Juday the weight of its 

 dry organic matter varies in three 

 Wisconsin lakes from slightly 

 less (rarely) to 15 or 20 times 

 more than that of the net plankton and is ordinarily 5 to 6 times 

 as great. This amount is unquestionably of marked importance 

 both scientifically and practically, and the character of the or- 

 ganisms indicates even more clearly their fundamental impor- 

 tance in the problems of aquatic biology. 



Plankton organisms are characterized by transparency, delicate 

 colors, and above all by their power of floating due *^.o buoyancy and 

 * New No. 25 is identical with No. 20 of older authors (Lohmann). 



Fig. I. A piece of bolting cloth No. 20 with 

 plankton organisms drawn between the meshes 

 to show relative size. Above, Rhizosotenia alata. 

 Upper row, left mesh: Gymnodinium, beneath 

 Ampltidmium rotundalum and ExuviaeUa baltica, 

 right Pouchctia parva; middle mesh: Prorocen- 

 trum micans and Rhynchomonas marina; right 

 mesh: Nitschia sigmatella. Achradina pulchra, 

 Halteria rubra, Nitschia doslerium. Middle row, 

 left mesh: Tintinnopsis nana, Tinlinnus steen- 

 strupi, Oxyrrhis phaeocysticola; middle mesh: 

 cham of small Chaetoceras species, above it on 

 the left Thalassiosira nana and saturni, on the 

 right Carteria; right mesh: chain of large 

 Chaetoceras species [Ckaet. didymum) , Tintinnop- 

 sis beroidea. Lower row, left mesh: Rliodomonas 

 baltica, Distepkanus speculum; middle mesh: 

 Strombidium caudatum (?), Meringospkaera medi- 

 terranea. Amoeba; right mesh: Coccolithopkora 

 wallichi, beneath on the left Pontosphaera huxteyi, 

 on the right Coccolithopkora leptopora, above on 

 the right Chrysomonadine without shell, at the 

 very bottom Rhabdosphaera claviger. X no- 

 (After Lohmann.) 



