134 SPOLIA ZEYLANICA. 



WINDOW-PANE OYSTER INVESTIGATIONS, 

 JANUARY AND MAY, 1913. 



By G. M. Henry. 



(With two Plates.) 



TN January last I went to Lake Tamblegam, primarily to 

 -*- obtain the larval stages of Placuna placenta, which is 

 supposed to spawn in the latter part of December or in 

 January. In addition to this I had arranged to make a 

 survey of the whole lake, to see how the large beds of one-year 

 old oysters which were discovered by Dr. Pearson in May, 

 1912, were thriving, and to take a series of measurements 

 which would throw further light on the growth-rate. When 

 I arrived at Tamblegam I found that the district was in a 

 state of flood owing to the abnormally heavy rains. On 

 January 12 a sample of plankton was taken by means of a 

 tow-net over the principal part of the bed from Kodaipota to 

 Sallaimunai, but the catch consisted entirely of freshwater 

 organisms and contained no Placuna larvae. The water tasted 

 quite fresh, and was full of light-brown sediment brought down 

 by the flooded rivers. Under such conditions it was useless 

 to expect to find larvae, and in consequence attention was 

 subsequently directed to inspecting the existing beds. A 

 preliminary line of nine dives was taken from Kodaipota to 

 Sallaimunai, and the oysters were found to be in a moribund 

 state. Few were actually dead, but nearly all were dying, 

 their valves tightly closed, mantle and foot fully extended. 

 Very little contraction could be induced by touching the 

 mantle or other sensitive parts. When actually dead the 

 valves gape and the pale appearance of all the organs clearly 

 indicates death. 



The three following days were spent in inspecting and 

 tow-netting with the same result — oysters rapidly dying 

 everywhere, except in the portion of the lake between Patai- 

 addimunai and Korrinjavat, where only a few were dead. 



