THE GILL OP THE CETLON PEARL-OTSTER. 227 



of the outer gill {g.f.) and the inner surface of the mantle-lobe 

 {Pall.). At these ciliated junctions the epithelial cells are cubical 

 or low columnar with, a distinct marginal band from which, tbe 

 very regular stiff cilia project (PI. 27. figs. 4 a & 5 a). 



It is interesting to note that in some, if not in all, specimens, 

 at the extreme ventral end of the median ciliated junction be- 

 tween the two inner gills, the epithelial and connective tissues 

 have united across the narrow passage (fig. 4, org.), and so true 

 concrescence or continuous organic union Has, at that point, 

 replaced the ciliated junction. 



In the Eulamellibranchia the ascending lamella of the outer 

 gill is usually concrescent with the mantle, and the inner lamell?e 

 of the inner gills are united in th.e middle line ; but such 

 junctions are quite exceptional in the Eleutherorhabda, and 

 where they do occur there is an irregularity about their distri- 

 bution, allied species showing very different conditions. Con- 

 sequently it is interesting to find this new form of junction, 

 which may be regarded as intermediate between perfect freedom 

 and complete concrescence *. The presence of this ciliated 

 form of junction, wbicb allows of an easy separation of the 

 opposed surfaces, may also account for the differences of opinion 

 we find amongst authorities. Pelseneer gives as a character 

 of the family to which the pearl-oysters belong that the branchiae 

 are joined to the mantle, and Eidewood says that in Meleagrina 

 vulgaris and M. margaritifera there is no fusion with adjacent 

 parts. My specimens, with their ciliated junctions, show the 

 one or the other condition according to their state of preservation. 



The second structural point I desire to record is the presence 

 of somewhat extensive organic connections between the adjacent 

 gill-filaments of a plica at the level of the ciliated discs. 



Figure 7 shows the manner in which, as seen in a horizontal 

 section across both ctenidia (four " gills," o.g. and i.g. on each 

 side), the filaments are thrown into crests and troughs so as to 

 form plicae. In the trough between two plicae lies a modified or 



* Eidewood (Phil. Trans. B. vol. 195, p. 194) has described, in the case of 

 Anomia aculeata, patches of interlocking cilia by means of which the lower 

 edges of the two inner gills are held together, and the lower edges of the 

 outer gills are joined to the mantle. The filaments in this case, however, are not 

 reflected, so the relations of parts are not the same as those described above 

 in the Ceylon Pearl-Oyster ; but the small patches in Anomia may be an indica- 

 tion of the same tendency to form cilialed junctions between adjacent parts. 



16* 



