THE CEYLON PEARL-OYSTER. aoe 
the pearl 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 pearl, at ¢. is a cyst-like hypostracum-pearl, comparable to the lining 
of the adjoining nacreous pearl. This hypostracum-pearl consistsof a cavity, 
lined with hypostracum-like substance (yp.’), and on two sides it shows 
muscle-attachment epithelium, m.ep.’’, but it has not acquired a lining 
of ordinary nacre-secreting epithelium, and hence no nacreous coats 
havearisen. ¢., cyst-like cavity of hypostracum-pearl; ep., shell-secreting 
epidermis; ep.’, epidermis of pearl-sac; m.ep., muscle-attachment epidermis ; 
m.ep.’, do. of pearl-sac; m.ep.’’, do. of hypostracum-pearl; m., muscle- 
fibres ; pav., parenchyma; hyp., hypostracam which lines the muscle 
pearl; hyp.’, do. of the hypostracum-pearl. Preparation IV,9. > 100. 
Fig. 9. A muscle-pearl in the free mantle, froma 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 enmposed 
of nacre at all parts but one.  o.ep., outer shell-secreting epidermis ; 
i.ep., inner ciliated epidermis of the mantle-cavity ; par., subepidermal 
parenchyma-cells, surrounding the pearl-sac; m.ep., muscle-attachment 
epithelium, where hilum of pearl was attached ; muse., muscles of mantle ; 
sac., pearl-sac; nac., nacre; rep.nac., granular repair-nacre, around central 
part of pearl; hyp., hypostracum. X 70. 
Prate XXXVI. 
Fig. 10. An early stage of a muscle-pearl, from one of Prof. Herdman’s slides. 
sac., pearl-sac; m., nacre; par., parenchyma; m., muscle-fibres; hyp., 
hypostracum-like cyst, around which the pearl is formed; m.ep., muscle- 
attachment epithelium; c., a hypostracum-pearl. x 65. 
Fig. 11. A ecyst-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; m., muscles ; par., parenchyma ; o.ep., outer shell-secreting 
epidermis ; ep.p.s., epidermis of pearl-sac. X 400. 
Fig. 12. Centre of the decalcified muscle-pearl which occupied the sac adjoining the 
hypostracum-pearl shown in fig. 11. ¢., central cavity; g7.. granules in 
central cavity ; ., 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 reinforcements ; J.c., lining of the central cavity. > 300. 
Prate XXXVII. 
Fig. 14. Four pearls “in ovary” 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, pay. ep., outer epidermis; ov., ovarian tubules ; m., muscle- 
bundles. Preparation XX VII, 8. > 30. 
Fig. 15. The centre of the pearl shown in the top left-hand corner of fig. 14. 
¢., central cavities ; nw., nucleus-like matter, probably of the nature of 
conchyolin, contained in same; 7vep.7., repair-nacre, with granular structure, 
tollowing upon the cavity; 2., ordinary nacre of the pearl. Preparation 
XXVIII, 8. > 400. 
Fig. 16. The central portion of the pearl shown on Plate XLIV. fig. 48. »., central 
«macre;, 2.’, peripheral nacre; c., central cavity; gr.d., granular layer, 
causing the opaque yellowish appearance of the central mass. 
Preparation LXXI s. xX 70. 
Puate XXXVIII. 
Fig. 17. A portion of the granular layer of the pearl shown in fig. 16, more highly 
magnified. m., central nacre; .’, peripheral nacre; g7., granular layer, 
‘suggesting dead cells; am.,am.’, two layers of amorphous horny substance. 
Preparation LXXI gs. x 400, 
Proc. Zoou, Soc.—1912, No. XXIII. 23 
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