134 CEYLON PEAKL OYSTER REPOliT. 



disc is the internal mould of the median pit, and the body of the thread that of the 

 laterally compressed tube formed by the approximation of the margins of the proximal 

 part of the pedal groove. The secretion forming the byssus hardens upon contact 

 with sea water. At first it is pale yellowish in colour and almost transparent ; in the 

 course of 48 hours it becomes opaque and of a deep bronze-green with extremely 

 lustrous surface. The sensitiveness to bright light of pearl oysters on the move after 

 sunset, when crawling or when forming new byssal threads, has been noted above. 

 When visiting the tanks it was noticed that no sooner did the candle flash upon them 

 than they either withdrew the foot instantly or did so the moment the thread they 

 were secreting was finished. Locomotion during daylight is very seldom seen, the 

 habit is undoubtedly to travel after darkness sets in a time when danger from 

 predaceous animals is no doubt reduced. 



Pearl oysters are equally sensitive to passing shadows during daylight. In the 

 aquarium tanks the hand passed slowly between them and the sun will cause them to 

 snap-to their valves ; and on the sea bottom , at a depth of 6 to 9 fathoms, the same 

 result of a passing shadow can be seen. It must be remembered that on the pearl 

 banks of the Gulf of Manaar the water is so clear that at the depths named the sea 

 bottom is brilliantly illuminated when a bright sun is shining. 



To some extent the habits of very young spat are exceptional. Thus when the 

 spat that adhere to floating weed are shaken off into an aquarium tank, they do not, 

 as older individuals would, lie quiescent on the bottom till after sunset, but at once, 

 whatever the condition of the light, start crawling up the sides of the tank, and fix 

 themselves on any objects well above the bottom. Probably an instinctive impulse 

 makes them endeavour to attain a higher level in order to avoid the risk of being 

 buried in the sand on the sea bottom. And it is remarkable what comparatively long 

 distances they cover within a few minutes. We have already shown (" Narrative," 

 p. 68) that minute spat from floating weeds can crawl at the rate of 1 inch a minute. 



IV. Food and Feeding Habits. 



The food of the pearl oyster is microscopic and of similar character to that of allied 

 Molluscs ; it consists largely of unicellular organisms spores of Algae, Diatoms, 

 lnfusorians, and Foraminifera in the main, with smaller numbers of Eadiolarians, the 

 minute embryos and larvse of various animals, and occasional considerable quantities 

 of delicate algal filaments (chiefly RhodophyceaB). Non-nutritious particles are also 

 met with in fair amount, such as the spicules of Alcyonarians and of sponges, with 

 small numbers of minute sand grains. On the whole, Diatoms, spores, and embryos 

 are the most important sources of food supply. 



Under natural conditions the pearl oyster lies with the right or less convex valve 

 beneath, and with the posterior edge of the shell elevated at an angle of about 20 

 degrees. If a detached oyster be placed on the bottom on its left or deeply convex 



