128 



CEYLON PEARL OYSTER REPORT. 



asymmetry, the anterior end being now rather sharper than the posterior (fig. 41, II.). 

 The velum was greatly reduced, occupying only a comparatively small space between 

 the anterior adductor muscle and the anterior margin of the gills. The gill filaments 

 were 6 to 7 in number and of larger size. A posterior adductor muscle was also 

 present, and the digestive gland, still markedly two-lobed, had increased in size. In 



Fig. 41. 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 Alga?. 

 III. has the mobile foot extended. 



Fig. 42. Stages in the growth of the shell after 

 the attachment of the larva. I. is identical 

 with the latest free-swimming stage ; o, 

 anterior, p, posterior end. II. shows the 

 first formation of prismatic shell (pr.). III. 

 and IV. show the change in shape and the 

 byssal sinus (hi/, s.). 



several of these larvse the foot was prominent and active, being frequently protruded 

 and moved about in a tactile manner ; a distinct prominence (the byssal gland) is seen 

 about the middle of the posterior edge (fig. 41, III.). The otocysts lie close to the base of 

 the foot, and each is a large clear sac containing 6 to 9 tiny otoliths which were in 

 constant vibratory motion. The shell is still clear and transparent, with no pigment 

 and no sign as yet of prismatic structure. 



From this stage, ready to make attachment, we pass to the earliest attached stage 

 ("spat"), where the little Mollusc begins to form shell layers of a different character. 



Our youngest spat was found clinging to the Hydroid Zoophyte Ca/mpanula/ria 

 juncea, in November, and later stages, still very young, on the Algae Hypnea 

 musciformis and species of Sargassum, Cladophora and Polysiphonia on the Cheval 

 Paar in February and March, 1902. The smallest stage, O'l millim. in antero-posterior 

 extent, is identical with the oldest free stage referred to above, and is presumably just 

 attached. From this to a shell length of O'l 75, increase is effected by marginal 

 additions which are clear and transparent like the larval shell. These additions show, 

 however, faint circumferential lines, but no prismatic appearance (fig. 42, 1.). The 

 dark digestive gland shows conspicuously in the umbonal region, and the rows of 

 branchial filaments are also visible through the thin shell. 



