582 Wisconsin Academy of Sciences , Arts, and Letters . 
At a speed of 300 revolutions per minute the capacity of 
each pump is about 30 liters per minute when the water is 
drawn through 30 m. (100 ft.) of hose having an inside diam¬ 
eter of 2.5 cm. (1 in.). The water is discharged into the 
large can and is strained through the large plankton net sus¬ 
pended therein. The effluent from the can discharges over 
the side of the launch. The arrangement of the pumping 
apparatus in the launch is shown in Plate XXXVI, fig. 1. The 
large plankton net was removed from the can so that it 
could be shown in the photograph. 
CENTRIFUGES. 
The plankton net retains only the larger organisms, such, 
for example, as the Crustacea, the insect larvae, the vast 
majority of the rotifers, and the greater part of the large 
forms of algae. While the meshes of the No. 20 silk bolting 
cloth are very small, especially after it has been shrunk, yet 
a very considerable portion of the plankton material is lost by 
the net. This assemblage of organisms which escapes through 
the meshes of the net is called the nannoplankton. It con¬ 
sists of such forms as rhizopods, flagellates, ciliates, rarely a 
rotifer, and various forms of algae chiefly the smaller ones. 
The rhizopods are represented by an occasional amoeba; 
the flagellates by the various monads, and an occasional Cer- 
atium, Peridinium, and Euglena; the ciliates by such forms as 
Paramoecium, Halteria, Coleps, and Vorticella. The algae be¬ 
long to two general groups, namely, those which are so small 
that they are regularly lost by the net and those which are 
lost only by accident. To the former belong such forms as 
Ankistrodesmus, Oocystis, Chodateha, Sphaerocystis, and 
some species of Goelosphaerium and Microcystis. Those lost 
accidentally are young individuals or colonies, individuals so 
small that they are readily lost through the meshes of the net 
when the catch is being concentrated in the plankton bucket, 
elongated or rod-shaped individuals which pass through 
when they strike the net endwise, and fragments of the larger 
colonies. To this group belong such forms as Anabaena, 
Aphanizomenon, Melosira, Stephanodiscus, Cylotella, Ta- 
bellaria, and Fragilaria. 
Various methods have been used for procuring the nanno¬ 
plankton organisms. Those that have been used most fre- 
