THE PARASITES OF THE PEARL OYSTER. 103 



EXPLANATION OF THE PLATES. 



PLATE I. 



Fig. 1. Planarian-like free-living organism, possibly the larva of a species of Cestode, taken in the 

 plankton on Muttuvaratu Paar, 19th November, 1902. It is most probably of the same 

 species as the encysted larvae found so abundantly in the pearl oysters of the paar named. 

 Actual length when elongated, 0-37 millim. The rudiment of what may be the proboscis is 

 already apparent, a-h, show various specimens in various attitudes; ex., calcareous 

 corpuscles; cut., the thick mucilaginous cuticular layer; Pr., possible rudiment of a proboscis. 



2. Three fragments a, b, and c of the distal or marginal region of the mantle of a pearl oyster, 



showing Cestode cysts scattered in the intermuscular connective-tissue. Natural size. 

 Mg.Pall., pallial margin : Mg.vel., velar margin; T.cy., Cestode cysts. 



3. A Cestode cyst (cy.) in an interlamellar junction at the base of a branchia. x 3 diameters. 



4. Fragment of a branchia of a pearl oyster showing encysted larval Cestode (cy.). x 12 diameters. 



5. a. Nucleus of a "fine" pearl from the posterior ear region of a Cheval Paar oyster. The 



proboscis sheath and the central pit within which the proboscis is retracted are clearly shown. 

 /.-. A " fine " pearl, showing distinct resemblance in outer form with the characteristic 

 appearance of a Cestode larva (see fig. 5, i), viewed anteriorly ; c, the same viewed laterally ; 

 d and e, natural size of the same. i. Outline of a young Cestode larva for comparison with 

 the preceding. /. An elongated pearl, also having resemblance in outline to a Tetrarhynchid 

 larva. h. Natural size of same. /. A lenticular pearl having an equatorial band of brown 

 prismatic pearl-substance. 



6. Section through an encysted larva of about the same shape as that represented in fig. i. The 



rostrum or proboscis (a) is still retracted in the body. 

 7. a and b, two of the later larval stages of the Cestode met with, in encysted condition, in the 

 tissues of the pearl oyster. Compare with fig. 13. x 80 diameters. 



8. A. A partially calcified Cestode cyst from the muscular pallial region of a 3-year old oyster 



from the Muttuvaratu Paar. The larva was dead and slightly changed in outline partial 

 disintegration having taken place. In colour it was dirty yellow. In the outer layers (b) of 

 the cyst capsule a deposit of lime salts had begun. B. Another cyst. b. The outer layers of 

 cyst ; c. Nucleus, obtained by decalcification, of a fine pearl from the muscular pallial region 

 of a pearl oyster from the Cheval Paar. C. Nucleus of another " fine " pearl from the ventral 

 pallial region of a pearl oyster from the same locality as before ; if. a few calcareous corpuscles 

 seen within the larva as solution brought the character of the nucleus into view. D. The 

 innermost of the membraneous laminaj left after solution of the corresponding pearl coats ; 

 c, a dead Cestode larva forming the actual nucleus; a ball-shaped calcification within the 

 Cestode larva is seen. (Weight 195 milligrs.) 



9. Very young Cestode larva extracted from a thick-walled cyst in the mantle of a pearl oyster. 



x 50 diameters. (Note. The outer layer is very faintly seen under a low power, and has an 

 appearance closely resembling ciliation. The distal limit is not so strongly marked as shown 

 here, see fig. 10.) c.c, calcareous corpuscles; Pr., proboscis; Pr.sk, proboscis sheath. 



.. 10. Young Cestode larva extracted from a cyst within the 'mantle of a pearl oyster. It shows 

 the minute denticulation of the proboscis collar. x 25 diameters. 



., 11. The same, seen under slight pressure, whereby the proboscis, Pr., has been evaginated forcibly. 

 Pr.sh., proboscis sheath. x 25 diameters. 



