356 ME. E. T. GtJNTHEE ON THE 



seemed to be fairly uniform, for even water drawn from a depth 

 of 28 feet contained its due proportion. 



In the month of August the vegetable portion of the plankton 

 -consists of small green masses, either of a globular or of a 

 membranous, flat and irregularly expanded form, of soft or 

 gelatinous substance, and varying from y^^ to | of an inch in 

 diameter. 1 at first regarded them as simple colonies of algse ; 

 but Mr. G. Murray, who has been kind enough to examine my 

 all too scanty material, assures me that their structure is that of a 

 bacterial zoogloea of micrococci invested by a number of small 

 diatoms. 



Their presence in such enormous quantities in the lake makes 

 me suspect that there may possibly be some more intimate vital 

 relations between the two organisms than would appear at first 

 sight. Their abundance in parts of the lake where there is 

 unlikely to be a proportionately large food-supply for plants of 

 holozoic nutrition, seems to indicate symbiotic relations between 

 the ehlorophyli-containing diatoms and the bacterial colonies, of 

 a nature very similar to those winch enable the constituents of 

 the lichen to maintain life in situations where life would be im- 

 possible without such a symbiosis. At the same time, it is 

 possible that the colonies may feed upon matter brought down 

 by the rivers, and that they may owe their universal distribution 

 to the surface-drift of the waters caused by the wind. 



The vegetable portion of the plankton aff'ords nutriment for 

 the fauna. As I have already stated in my letter to ' Nature ' of 

 Sept. 8, 1898, the so-called "jellyfish" alluded to by Lord 

 Curzon of Kedlestone and Mr. P. L. Sclater is a species of 

 brine-worm allied to Artemia salina, Leach. Li the shallows 

 near the muddy shores are to be found the aquatic larvse of a 

 Dipterous insect not unlike the larvae of JEpliydra riparia. As 

 in the rat-tailed larvse of JSristalis and of PtycJioptera paliidosa, 

 the respiratory tubes are prolonged posteriorly and admit of con- 

 siderable extension, so that the larva is able to draw air into its 

 tracheal system while crawhng in search of food beneath the 

 surface. The extremity of the respiratory tube is forked, and 

 each branch is tipped with small hairs which naturally increase 

 the clinging power of the apparatus to the surface-film of the 

 water. The larvae were about 10 mm. in length. 



The little brine-worms (At'femia urmiana) were as a rule uni- 

 formly distributed throughout the lake, but clouds and streaks 



