Uo. 6 (1921) COMMON MOLLUSCS OF SOUTH INDIA I63 



from a glue-like secretion poured into a fine groove on the under- 

 side of the tongue-shaped foot. On contact with water it hardens 

 into a strong, slightly elastic substance, golden yellow in young 

 oysters, dark bronze green in adults. 



The sexes are separate, and fertilization of the tiny ova, of 

 which several millions are produced by one individual, takes place 

 in the water. Development is rapid, a trochosphere stage first and 

 then a veliger. In the former stage, the larva is a little sphere 

 with a ring of rhythmically waving tiny threads {cilia) around the 

 equator, and a tuft of long filaments from the " North Pole " of the 

 little creature. The lashing of this ring of cilia carries the larva 

 through the water rotating on its axis. In the veliger a tiny shell 

 with triangular valves has been formed, but still for some days yet 

 the little creature continues its free-swimming existence near the 

 surface of the sea, a ring of cilia, the veliun, in front of the mouth, 

 forming the swimming organ. At the end of about a week or even 

 earlier if conditions be favourable, the larva settles down to the 

 bottom and makes its first attachment. In this condition it is 

 called "spat," the earliest stage of the settled period of its life. It 

 is obvious that a satisfactory "spat-fall" on the pearl-banks is 

 dependent upon a favourable run of currents at the time the oysters 

 spawn ; experience shows this to be the exception rather than the 

 rule ; there are many years when no extensive spat-fall occurs and 

 hence pearl-fisheries in the Gulf of Mannar, both on the Indian 

 and the Ceylon sides, are irregular. Blank years are more frequent 

 than prolific years, especially on the Indian side. Worst of all, 

 science is powerless ; no human agency can control ocean currents 

 and it is their influence and interaction more than any other factor 

 that determine the conditions favourable or otherwise for an exten- 

 sive spat-fall. 



Pearl-oysters occur sporadically all the way along the East 

 Coast from Cape Comorin to Madras. I have even found a young 

 living individual in Pulicat backwater. But the only places where 

 they occur in fishable quantities are the Gulf of Mannar and Palk 

 Bay. The "banks" off Tuticorin have been worked from time 

 immemorial, certainly for over 2,000 years. Those in Palk Bay are 

 a recent discovery, for they were found as recently as 1913 during 

 an inspection carried out by the writer. In the Gulf of Mannar 

 pearl-oysters occur on banks where there is a good deal of stony 



