OBSEEVATIONS ON THE VOLVOCACEAE. 335
bottom of the dish ; and inside each case was a Ghironomus larva which escaped out on being disturbed with a needle. An examination of a green case showed that it was made up of large numbers of Pandorina colonies. The larva on being examined under the micros- cope showed that it had been feeding on these Pandorina colonies ; for its alimentary canal was full of them.
Another curious phenomenon noticed by me was a tiny unicellular Blue Green Alga attached to the body of a Ohlamydomonas. This Blue Green Alga was evidently getting a free ride at the expense of the Ohlamydomonas and benefiting by the advantages of the locomotion, viz., better aeration and better food supply. Often there was more than one rider on a single Ohlamydomonas, sometimes as many as five or more. I occasionally came across a few cases where the riders were too many in number for the poor Ohlamydomonas to carry, which was therefore unable to move at all.
Summary
The following members of the Volvocaceae have been collected from Madras : --Ohlamydomonas, Carteria, Gonium, Pandorina, E ado - rina Pleodorina and Volvox. The last two are generally very rare forms.
Though thase forms may occur in various parts of the year and in various situations, they attain their greatest luxuriance only in the many small rain-water pools formed by the showers of the Summer Monsoon season which at Madras are generally light and irregular, though frequent.
The organisms seem to like light of moderate intensity. They come up sometime after sun-rise and if, in the middle of the day, the sun-light is too strong, recede below and come up again towards the latter part of the day and form a thick green film on the surface. In this condition the organisms are generally very inactive. They sink down again during the night. If the weather is cloudy they do hot sink down in the middle of the day, nor do they float up, towards the evening to form a film, but are active throughout the day.
In certain rain-water pools formed in the Madras Beach, these organisms are often found in large numbers on the wet sand around the pool giving it a green colour. This situation is sought by them probably for securing better aeration. In one of these pools where the water had disappeared the wet sand was greenish, and its greenness was due to many quiescent Chlamydomonadine cells without cilia, This invasion of a sub-aerial region by a free swimming aquatic form suggests how some sub-aerial unicellular Green Algae might have originally evolved from some free swimming Chlamydomonadine type.
