126 CEYLON PEARL OYSTER REPORT. 



kept under observation from season to season, and formed a part of the work which I 

 left Mr. Hornell to settle at Galle. 



On May 17th, 1902, four of the pearl oysters in our aquarium were found to be 

 emitting sexual elements, three being determined as males and one as female. They 

 were marked individually and put in a separate tank, and later in a cage suspended 

 from a buoy in the bay. On September 23rd two of these verified males were 

 brought into the laboratory and both proved, on dissection (October 1st), to be still 

 in the male condition. Over four months had elapsed since the end of the observed 

 emission of sperms, and the gonads had filled up again for the next breeding season. 

 The larger individual especially showed vigorous growth in all parts, the shell had 

 increased from to -fa inch, in addition to a great development of marginal processes. 

 The gonad was fully developed, creamy yellow in colour, and swollen with active 

 spermatozoa. The two remaining verified individuals were brought in and dissected 

 on October 3rd, and were found after the 4|- months' interval to be still of their 

 respective sexes. The male contained ripe spermatozoa, and the female had the 

 gonad large and swollen with ova in exactly the same condition as those emitted in 

 May. Spawning might have taken place within a day or two had they lived. 



Mr. Hornell has observed the natural emission of the eggs in a 2^- years old pearl 

 oyster living healthily in the aquarium tanks of the Galle Laboratory. He was also 

 able, by pouring some of the sea-water charged with freshly laid ova into a tank 

 containing other pearl oysters, to stimulate three ripe males to discharge spermatozoa. 

 The first male commenced within two minutes of the addition of the ova to the tank, 

 and while the stream issued as a milky cloud from the exhalent orifice the animal 

 at intervals snapped its valves together several times, thus effectively dispersing the 

 spermatozoa through the water. One minute after the first a second male commenced 

 to spout and continued to pour out a stream for six minutes. After an hour's interval 

 it re-commenced for four minutes, followed by occasional puffs for nearly half an hour, 

 when the emission ceased finally. The third male became active some hours later, 

 but all proceeded in a similar manner. 



These observations show that the ova are extruded direct from the oviducts, by 

 means of the supra-branchial passage, to the sea. They do not pass any time within 

 or upon the gills and do not undergo fertilization within the parent. The develop- 

 ment of the larva takes place entirely in the sea. 



II. Embryology and Early Life-History. 



We tried artificial fertilization on several occasions, and although we have not yet 

 reared the pearl oyster from the egg to a young shelled Mollusc that would be 

 recognised by the divers, still we have had, in our tanks and under our microscopes, 

 all stages from the fertilized ovum to free-swimming larvse, similar to those we caught 

 in the tow-net on the pearl banks, and which again we traced into the attached spat 



