Volvox
are colonial flagellates and a very popular organism for classroom
observations. The colony is large, measuring from 100-6000 microns
across. The
colony is comprised of many single, bi-flagellated cells connected
together by protoplasmic strands. It forms a hollow, green
sphere. Individual cells have a red eyespot and, with chloroplasts,
can make food from sunlight. Daughter colonies grow within this main
colony and eventually break free and develop as a parent colony. Small
animals like rotifers prey on the Volvox. The cells are vegetative (i.e.
they do not reproduce) except for a few near the back, which develop into new
colonies. Eventually the parent bursts and the daughter colonies evert.
Sexual reproduction is oogamous - one gender produces small motile gametes and the other non-motile
gametes.
Volvox is found in ponds and ditches, and even in shallow
rain puddles. Volvox
are readily available from science supply companies. 
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