136 CEYLON PEARL OYSTER 



EXPLANATION OF PLATE. 



Figs. 1 and 2. Living eggs of Margartiifera vulgaris, immediately after emission. 



3 and 4. Preserved ovarian eggs, as seen in sections of the mature gonad. x 900. 

 Fig. 5. The spermatozoon, highly magnified. 

 Figs. 6, 7, and 8. Segmentation stages. 



[The exact magnification of these figures, drawn in Ceylon, is not known.] 



Fig. 9. Segmentation nearly completed, embolic gastrula. Fig. 10. Optical section of the same stage. 

 11. Embryo showing first appearance of the zonal band of cilia, and differentiation of posterior end. 

 12. Optical section of the same stage. 

 13. Early trochophore stage, 22 hours after fertilisation. 

 14. Trochophore four hours later. 

 15. Optical section of the same stage, a, prostomium; b, apical tuft of cilia; c, prse-oral circlet of 



cilia ; d, blastopore ; e, enteron ; /, posterior ciliated patch. 

 16. Trochophore four hours later 30 hours after fertilisation. 

 17. Two unfertilised ova, three days after extrusion. 



18. Free-swimming larva, three days after fertilisation, showing the first appearance of the shell. 

 ,, 19. Free-swimming larva caught along with surface plankton in the tow-net. 

 20. An older form when ready to settle down as the young fixed pearl oyster. 

 21. Another larva of about the same age, showing especially the otocyst (enlarged at 21a), and the 



long mobile foot /, which can be extended far beyond the shell. 



Figs. 22 to 27. Stages of growth in the shell of the fixed spat, from 0'4 millim. to 1'5 millims. across. 

 Fig. 22 shows practically the same stage as fig. 19, although the latter was free and the 

 former fixed. In fig. 27, p. indicates the prodissoconch, s. the prismatic shell, and n. the 

 portion lined by nacre. 



28 and 29. Two views of the hinge, showing the simple, regular, interlocking teeth. 

 Fig. 30. Part of the shell in the 6-millim. stage (fig. 23), showing the characters of the prodissoconch 



(p.\ of the prismatic shell (s.), and of the cuticular margin (in.), x 900. 

 31. Very young spat (0 4-millim. stage) on filamentous Algae. Natural size. 

 32. Older spat of various sizes on Algaa. Natural size. 

 33. " False spat " (Avicula vexillum) on Algae. Natural size. 

 34. Young Aviciila vexillum (" false spat "). x 40. 



Fig. 19. Bank of " window-shell " oysters (Placuna placenta), remaining from the last fishery 



in Lake Tampalakam, at Trincomalee. 



