ANATOMY OF THE PEARL OYSTER. 71 



Pris., prismatic layer of shell. Sn.tr., transverse sinus. 



/'.'./'., postcro-ventr.il fold of stomach. S.o., pallial sense organ. 



Ret., pedal retractor muscle. S.ce.p., sub-o3sophagcal pit. 



Bet.', impression of retractor muscle. St., stomach. 



Ret.ins., insertion of retractor muscle. St. 1, intestinal or pyloric chamber of stomach. 



Ret.pail., pallial retractor muscle. St. 2, right or caeca] chamber of stomach. 



Rn.o., renal orifice. Ty., typhlosole. 



Hit. per., reno-pericanlial canal. V., ventricle. 



Rn.pi /.', aperture of last. Vel, velum of mantle. 



Rn.s., renal sinus. Vel.pr., velar processes. 



R.r., rectal ridge. /"./., visceral lobe. 



Sn.add.lat., lateral adductor sinus. V.m., visceral mass. 



Sii.iit., median sinus. 



PLATE 1. 



Fig. 1. Right valve of a specimen of the Ceylon pearl oyster (MargarUiferu vulgaris) horn Kondatchi Paar, 



showing an unusually smooth and rounded form ; nat. size. This seems very similar to the 



'Shark's Bay shell," described by Jameson as M. careharia/rum. 

 ,, 2. Light valve of another specimen of the same species brought up in the same haul from Kondatchi 



Paar, to illustrate variation in the shell ; nat. size. 

 ,, 3. Inside of right valve of an unusually wide specimen from Kondatchi Paar, showing a broad 



margin devoid of nacre and very large auricles ; nat. size. 

 4. Inside of left valve of a more normal shell, from Cheval Paar, showing well-developed nacre and 



a group of small pearls near the anterior auricle ; nat. size. 

 ,, 5. Outside of left valve of an unusually straight and narrow specimen, from Cheval Paar, the surface 



covered with Polyzoa and other organisms ; nat. size. 

 ,, 6. Outside of left valve of an unusually oblique specimen from Cheval Paar, the surface largely 



covered with adhering organisms ; nat. size. 

 (Figs. 1 and 2 from drawings by Mr. J. Hornell ; figs. 3 to G from photographs by W. A. H.) 



PLATE II. 



Figs. 1 and 2 show photographs (W. A. H.) of the inside and outside of the valve of a normally developed 



pearl oyster, showing form, markings and nacre ; slightly reduced. 

 Fig. 3. From a photograph (J. H.) of the animal after removal of the left valve, showing foot and byssus, 

 mantle, gills, adductor muscle, \'c. ; nat. size. 

 4. From a photograph (J. H.) of pearl oysters from (A) Muttuvaratu Paar and (B) East Cheval 

 Paar, of the same age (3 years to 3J years) to show the marked difference in growth ; 

 nat. size. 

 ,, 5. From a photograph (J. H.) of three pearl oysters seen edgeways, to show adductor muscle and 



mantle lobes ; nat. size. 

 ,, 6. From a photograph (J. H.) of a pearl oyster with the right valve removed and dissected to show 

 the adductor muscle, foot, &c. ; nat. size. 



PLATE III. 



Figs. 1 to 5 show differences in the soft parts of three species of " pearl oysters." 



Fig. 1. Anal funnel of Marga/ritifera sp. (?) a small, flat, dark brown species, A in ventral and B in 

 lateral view; enlarged; a., anus. 



,, 2. Ventral part of visceral mass of same species from the side ; nat. size. 



3. Anal funnel of M. vulgaris (SCHTJM.), A in ventral and B in lateral view; enlarged; a., anus. 



