THE GULL OF THE CEi'LON PEARL-OYSTER. 229 



Organic interfilamentar j unctions are characteristic of the 

 Eulamellibrauchia, and, as Eidewood has noted, " with one 

 exception do not occur in the Eleutherorha.bda." It is therefore 

 of some interest to add tlie Ceylon Pearl-Oyster as another 

 exception, and to find that in this character of the interfilamentar 

 junctions, as well as in that taken from the connections of the 

 gills with neighbouring parts, this member of the Eleutherorhabda 

 shows an approach to the Eulamellibranchiate condition. 



EXPLANATION OF PLATE 27. 



Fig. 1. Diagrammatic transverse section through the two ctenidiaof the Pearl- 

 Oyster, to show the axes (ci.a.) and the position of the median (M.c.j.) 

 and lateral (L.c.j.) ciliated junctions. X 2. 



2. Dissection of left ctenidiam showing the appearance of the median and 



lateral ciliated junctions. X 2. 



3. Transverse section through left mantle-lohe and botli ctenidia, to show 



the extent of ihe median and lateral ciliated junctions. X 15. 



4. The median ciliated junction between the inner lainellEe of the inner 



gills: or^., slight organic connection. X400. 4 a. Small part of the 

 ciliated epithelium, enlarged. 



5. The lateral ciliated junction between the outer lamella of the outer gill 



and the mantle-lobe (Pall.). x400. 5 a. Small part of the ciliated 

 epithelium, enlarged. 



6. Longitudinal section along the gill-filaments, to show the ciliated discs 



(cd.). X 50. 



7. Horizontal section across the two ctenidia (four "gills," o.g. and i.g.). 



X 50. 



8. Part of last more highly magnified to show the filaments in cross- 



section. X 400. 



9. An ordinary gill-filament in transverse section. X 500. 



10. Transverse section of a filament at the level of a ciliated disc (c.d.). 



X 500. 



11. Section showing two filaments joined by organic union at the level of a 



ciliated disc. X 400. 



12. Section showing a group of filaments joined by various combinations 



of organic union vrith ciliated discs. X 400. 



