GENERAL SUMMARY AND RECOMMENDATIONS. 



115 



Fig. 4. Sketch of young pearl-oyster spat 

 attached to Sargassum. 



shell, were found during March and April, 1902, attached to both rooted and floating 



Algae in various parts of the Gulf of Manaar. The so-called " false-spat " (other 



smaller allied shell-fish, such as species of Avicula) also occurs on Zoophytes and 



Algae ; but during the time of our investigations there was undoubtedly abundance 



of the true pearl-oyster spat both on 



filamentous green and red Algae from 



the bottom (Plate, figs. 32, 33), and 



also on floating Sargassum weed (fig. 4). 



The importance of these Algae in thus 



affording attachment to the youngest 



stage of the spat, and in afterwards 



distributing it widely, can scarcely be 



over-estimated (for the names of the 



species of Algae involved, see ' Report 



on the Algae,' Part I., p. 163). 



Mr. HORNELL has been unable, since 

 he became Inspector of the Pearl Banks, 

 to obtain, during the anxious and busy 

 periods of successive inspections and fisheries, the amount of free time necessary in 

 order to make detailed observations on the embryonic development ; but we give 

 here a brief outline, illustrated by a series of figures (see Plate), most of which he 

 made in the summer of 1902 from embryos reared at the Galle Biological Station. 

 Figs. 1 and 2 show the living egg on extrusion to be provided at one end with a 

 micropyle through which fertilisation takes place, and which may be prolonged (fig. 1) 

 as a slender tube. Figs. 3 and 4 represent the mature ovarian egg, fixed and stained 

 as we now find it in our sections, and showing a well-marked nucleolus lying in a 

 clear vacuole. The egg shown in fig. 4 measured about 0'05 millim. along its greater 

 axis. Fig. 5 gives the outline of the spermatozoon much more highly magnified 

 than the ova. 



The segmentation is complete, but unequal, and the stages seen in figs. 6, 7, and 8 

 agree with those we are familiar with in some other allied molluscs. The enclosure 

 of the larger macromeres by the smaller micromeres, seen in progress in figs. 7 and 8, 

 is shown far advanced in 9 and completed in 11. The single macromere of fig. 8 has 

 divided into two in 9 (see optical section 10) and into four in 11 (see optical 

 section 12). Fig. 11 shows the flattening of the lower, posterior, end and the first 

 appearance of a zonal (prae-oral) band of cilia round the widest part of the body. 

 This is a young trochophore stage, and completed trochophores are seen in figs. 13 

 (22 hours), 14 and 15 (26 hours), and 16 (30 hours after fertilisation). These stages 

 show an enteron opening to the exterior near the posterior end, a posterior patch of 

 cilia, a long anterior tuft on the prostomium, and an equatorial prae-oral circlet of 

 cilia (see fig. 15). 



Q 2 



