THE WINDOW-PANE OYSTERS. 35 



mud, some of which have been colonized by window-pane oysters. 

 Such a recess occurs at the mouth of thelrattamaddikkali-odai, shut 

 off from the main current by a long mangrove eyot or " nadutivu." 

 A small quantity of shells thrown upon the bank indicated what had 

 been taken last year. It may be stated at once that the placuna 

 fishery is not an important industry in the Sambore river and hardly 

 shows promise of becoming one, but the occurrence of placuna here 

 is an important biological fact for several reasons. 



In his 1905 report, to which I have referred above, Mr. Hornell 

 describes his original investigations into the causation of pearls in 

 Placuna placenta, and announces his discovery that the same larval 

 cestode which stimulates the formation of pearls in the Mannar 

 pearl oyster {Margaritifera vulgaris), also furnishes the nucleus 

 of the pearls produced by placuna. Mr. Hornell remarks upon the 

 singularity of this parasitological uniformity in divergent environ- 

 ments. The tidal Sambore river introduces us to a third class of 

 conditions of existence, inasmuch as instead of opening to the sea 

 by a wide and comparatively deep strait as does lake Tamblegam, 

 the discharge of the river takes place over a shallow surf-ridden 

 sand bar. Nevertheless, the parasitological conditions remain ap- 

 parently the same in all essential respects. Later on I shall have 

 something more to say upon the subject of the multiplication and 

 migrations of the parasitic larvse, which are so abundant in the liver 

 of these bivalve molluscs. Following the usual custom I now pass 

 on to the enumeration of the various stations where samples were 

 collected during my first inspection. 



Station I. — At the seaward extremity of Irattamaddikkali-odai ; 

 depth 2-4 feet. In spite of the shallowness the water contained so 

 much matter in suspension that the bottom could not be' seen from 

 the boat, and the divers waded about depending upon the tactual 

 acuity of their feet to find the shells. This method serves well 

 enough for moderately large specimens in shallow water, but is not 

 sufficiently delicate for the young thin-shelled stages. In the course 

 of about half an hour only six living placunaj were taken, as shown 

 in the subjoined table, the measurements being made with the 

 callipers on a scale of millimetres : — ■ 



Length. Height. Remarks. 



1 .. 158 .. 143 .. Pearls found in the right mantle 



only, a cluster of small pearls 

 inside the mantle opposite the 

 labial tentacles. Copepod ecto- 

 parasites with briglit roseate 

 ovisacs glided rapidly over the 

 gills. 



2 .. 148 .. 135 .. Several of the red Copepod Crus- 



tacea present. 



