92 



LIFE TIISTOKY OF THE PEARL OYSTER. 



The oysters are of separate sex, male and female. The eggs of the 

 female, when ripe, are extruded into the sea water, where they are 

 fertilized by the spermatozoa of the male, if by chance the currents bring 

 the two elements together. Doctor Hornell ^ observed in regard to the 

 Ceylon pearl oyster, "that a ripe female, in close proximity to a mature 

 male, was sufficient cause to excite the male to throw off spermatozoa." 

 The meeting of the spermatozoa and ova is left entirely to chance. 



The, eggs of the Philippine pearl oyster {M. viaxima Jamson) are at 

 first pyriform (see fig. 1) and float on the water; as soon as they are 



fertilized, they become round ; 

 and when from three to six 

 hours old, they move about 

 Ijy means of small, hair-like 

 cilia. Segmentation is com- 

 plete, but unequal. The shell 

 begins to form at the end 

 of the second day, and' in 

 from four to eight days the 



Fig. 1. — Spermatozoa and ova of Philippine pearl 



oyster. young oysters settle and be- 



en) Micropyle through which the spermatozoa ^^^^^^^ attached tO the bottom, 

 enters the ova. 



(6) Nucleus. or to any object they chance 



(c) Nucleolus. to fall upon. At this stage 



(d) Spermatozoa of male. ^ 



they are known as spat and 

 are about 1 millimeter (0.04 inch) in length. They attach by means 

 of a small tuft of coarse, hair-like bristles, known as the byssus. 



Each mature female contains several thoiisand eggs, but no doubt the 

 destruction of eggs and young is very great, many being swept into great 

 depths by strong currents, where they either perish or settle on the 

 bottom so thickly as to smother each other; or else they become covered 

 with sand, or attach to some floating object and are washed ashore. 

 Apparently, there is no fixed time in the Philippines for the maturing 

 of the ova, as sexually ripe individuals are found at all seasons of the 

 year. 



The shells are supposed to reach a maximum size and are most valuable 

 commercially in from four to five years, although tliey doubtless con- 

 tinue to grow for several years longer. I have examined specimens 

 weighing 5 kilos (11 pounds) which I believe were ten to twelve years old. 

 However, shells older than five or six years are apt to be worm-eaten 

 or full of holes caused by boring -sponges, and so are of less value. More 



° Report on the operations of the Ceylon pearl banks during the fishery of 

 1905. Rep. Ceylon Marine Biol. Lai. (1906), 55. 



