11(5 



CEYLON PEARL OYSTER REPORT. 



Fig. 5. Free-swimming larval stages of pearl oyster caught in 

 the tow-net. I. has the ciliated velum retracted. II. and III. 

 show the stage at which the larva becomes attached to Algre. 

 III. has the mobile foot extended. 



After this stage our series is not so complete, but we give such stages as we have 



of later larvae, as they may be useful to future observers for comparison with forms 



captured in the tow-net. Fig. 17 shows a couple of unfertilised eggs, not yet 



beginning to decay or disin- 

 tegrate, at three days after 

 extrusion from the parent. 

 Fig. 18 shows the first appear- 

 ance of the larval shell, three 

 days after fertilisation ; and 

 figs. 19, 20, and 21 show three 

 pelagic (also shown in text- 

 fig. 5), but shelled, forms. The 

 ciliated velum (a), the adductor 

 muscles, the mobile foot, the 

 otocyst, a pigmented eye-spot, 

 the developing gill filaments, 

 &c, are readily seen. 



The larva is now at a stage 



when it is ready to affix itself to some foreign body (Plate, fig. 32), such as the 



filamentous green Algse. Figs. 22 to 27 (and also text-fig. G) show young stages of 



growth of the shell after fixation. Fig. 22, the youngest fixed form we have found 



adhering to Alga?, measures 0'4 millim. in 



greatest extent, and is identical with 



fig. 21, the latest free-swimming stage we 



have found. Fig. 23 shows that the new 



growth added to the shell (" prodisso- 



conch ") of the " spat " alter fixation is 



formed of prismatic substance, and is 



entirely different in appearance from the 



structureless embryonic shell marked only 



by concentric lines of growth. Fig. 31 



shows the junction of these two layers 



of the shell, and also the free margin, 



magnified, at this stage. Further additions 



of prismatic substance which is gradually 



surrounding the embryonic shell are shown 



in figs. 24 and 25. The byssal sinus, 



indicating the anterior end of the shell, appears in these stages. In the next (fig. 26) 



it has worked its way further dorsally. Developing gill filaments are present in all 



these stages, seen through the thin shell of the "spat." In fig. 2G the filaments have 



become long and slender, and are about 10 in number. The spat is now 1 millim. 



Fig. 6. Stages in the growth of the shell after the 

 attachment of the larva. I. is identical with the 

 latest free-swimming stage : a, anterior, p, pos- 

 terior end. II. shows the first formation of 

 prismatic shell (pi.). III. and IV. show the 

 change in shape and the byssal sinus (by.s.). 



