Genus: Highly flattened; pellicle firm; body form constant (Kudo, 1966). Chloroplasts small, discoid, pyrenoids usually absent; most species flat and leaf-shaped; often with ridges or fins running helically or longitudinally (Illustrated Guide, 1985).
Species: Cell body ovoidal or ellipsoidal, 30-50 μm long, 15-20 μm wide, cauda nearly straight or slightly curved, keel reached at the base of the cauda, striae straight, paramylon bodies disc-shaped with a hole at its center, 1-3 in number (An Illustrated Guide to Freshwater Zooplankton in Japan, 2000).
Google Image Search
P. caudatus
28 x 16 μm
Hiroshima, 1998
Movies 
P. caudatus
21 x 12 μm
ruguay, 1999
P. caudatus
μm
Mizuko-kaizuka p.
Fujimi
Saitama, 2004
P. caudatus
32 x 16 μm
Kiyasse-Hanyu
Hanyu
Saitama, 2006
P. caudatus
35 x 20 μm
Hazama marsh
Ashikaga
Tochigi, 2006
