THE CEYLON PEARL-OYSTER. ;{5;{ 



tliepeavl consists of typical coats of nacre ; but at the point where the sac 

 IS lined with attachment-epithelium the pearl consists of hypostracum, 

 which also (or a substance analogous to it) lines the central cavity. Beside 

 the peaii, at c. i.s a cyst-like hypostracum-pearl, comparable to the linino- 

 ot the adjoiiiine,- nacreous pearl. This hypostracuin-pearl consistsof a cavitj" 

 lined with hyiiostracum-like substance {Iryp.'), and on two sides it shows 

 muscle-attiicbmcnt epithelium, m.ep.", but it has not acquired a lining 

 ot ordinary iiacie-secreting epithelium, and hence no nacreous coats 

 have arisen, c, cyst-like cavity of hypostracum-pearl ; ep., shell-secreting 

 epidermis ; ep.', epidermisof pearl-sac ; in.ep., muscle-attachment epidermis ; 

 mep.', do. of pearl-sac; m.ep.", do. of hypostracum-pearl; m., muscle-' 

 fibres; par., parenchyma; hyp., hypostracum which lines the muscle 

 pearl ; /i.y^.', do. of the hypostracum-pearl. Preparation IV, 9. X 100. 

 r ig. 9. A muscle-pearl in the free mantle, from a slide in Prof. Herdman's collection. 

 The area where the muscle-attachment epidermis is attached to the pearl 

 is relatively very small. The greater part of the pearl-sac is composed of 

 ordinary nacre-secreting epidermis, and consequently the pearl is composed 

 of nacre at all parts but one. o.ep., outer shell-secreting epidermis ; 

 i.ep., inner ciliated epidermis of the mantle-cavity; ^ar., subepidermal 

 parenchyma-cells, surrounding the pearl-sac ; m.ep., muscle-attachment 

 einthelium, where hilum of pearl was attached ; muse, muscles of mantle ; 

 srtc, pearl-sac ; nac, nacre; rejii.jine., granular repair-nacre, around central 

 part of pearl; hyp., hypostracum. X 70. 



Plate XXXVI. 



Fig. 10. An early stage of a muscle-pearl, from one of Prof. Herdman's slides, 

 sac, pearl-sac; »., nacre; par., parenchyma; m., muscle-fibres; hyp., 

 hypostracum-like cyst, around which thepearl is formed ; m.ep., muscle- 

 attachment epithelium ; c, a hypostracum-pearl. X 65. 



Fig. 11. A cyst-like hypostracum-pearl in the muscle-bundle, without any apparent 

 epithelial sac. From one of Professor Herdman's slides. The cyst adjoins 

 the wall of a sac, ep.p.s., in which a muscle-pearl was contained, hy., 

 hypostracum; ?«., muscles ; par., parenchyma ; o.e/>., outer shell-secreting 

 epidermis ; ep.p.s., epidermis of pearl-sac. X 400. 



P'ig. 12. Centre of the decalcified muscle-pearl which occupied the sac adjoining the 

 hypostracum-pearl shown in fig. 11. c, central cavity ; ^r., granules in 

 central cavity ; n., nacre. X 500. 



Fig. 13. Centre of a decalcified pearl, close to the mantle-margin, in one of Prof. 

 Herdman's slides, c, central cavity; «,., nacre; rep.n., repair-nacre with 

 radial reiufoicemeiits ; I.e., lining of the central cavity. X 300. 



Plate XXXVII. 



Fig. 14. Four penrls " in o\ ary " of Margaritifera vulgaris, from Dr. Kelaart's 

 material in the British Museum. The pearls are close together as shown, 

 but their centres have been brought into the same plane in the drawing to 

 show the " nuclei." Each pearl is surrounded by a mass of subepidermal 

 parenchyma, par. ep., outer epidermis; oo., ovarian tubules ; m., muscle- 

 bundles. Preparation XXVll, 8. X 30. 



Fig. 15. The centre of the pearl shown in the top left-hand corner of fig. 14. 

 c, central cavities; nu., nucleus-like matter, probably of the nature of 

 conchyolin, contained in same ; rep.n., repair-nacre, with granular structure, 

 following upon the cavity ; «., ordinary nacre of the pearl. Preparation 

 XXVII, 8. X 400. 

 Fig. 16. The central portion of the pearl shown on Plate XLIV. fig. 48. w.. central 

 nacre; «.', peripheral nacre; c, central cavity; gr.l., granu'ju- layer, 

 causing the opaque yellowish appearance of the central mass. 

 Preparation LXXI j. X 70. 



Plate XXXVIII. 



Fig. 17. A portion of the granular layer of the pearl shown in fig. 16, more highly 

 magnified. »., central nacre ; «.', peripheral nacre ; gr., granular layer, 

 suggesting dead cells ; am., am.', two layers of amorphous horny substance. 

 Preparation LXXI j. X 400. 



Pnoc. ZooL. See— 1912, No. XXIII. 23 



