228 PROF. W. A. HEEDMAN ON THE STRUCTURE OF 



" principal " filarneut (figs. 6, 7, 8,p.f.). At certain levels (fig. 6) 

 the ordinary filaments are kept in position by pad-like thickenings, 

 the ciliated discs (c.d.) bearing short stiff" cilia which interlock. 

 Dr. Eidewood, in his recent admirable memoir " On the Structure 

 of the Grills of the Lamellibranchia " (Phil. Trans. B. vol. 195, 

 1903), has figured in Avicula argent ea the simultaneous presence 

 of ciliated junctions and true organic union of the filaments ; 

 and in stating that no other instance occurred in the whole 

 of his extensive investigation, adds (p. 1 55, footnote), " except 

 perhaps in Meleagrina vulgaris, the specimen of which was not 

 sufficiently well-preserved to enable one to determine the point." 

 The latter species is the Ceylon Pearl-Oyster and the Avicula 

 is a closely allied species ; and I can entirely agree with all that 

 Dr. Eidewood has been able to determine in regard to these 

 forms. All I am able to do is to add a little detail, and even that 

 point of structure was probably suspected by Dr. Ridewood, 

 although his material was not apparently in a condition to show 

 it. In his memoir (p. 213), after describing the condition in 

 Avicula argentea, he says: — "Possibly a similar cellular con- 

 nection obtains in Meleagrina vulgaris.''' I can now state 

 definitely that it is in that species, as Dr. Ridewood thought might 

 possibly be the case. In examining a series of sections through 

 the plicae of well-preserved gills, it is easy to find groups of 2, 3, 

 4, or 5 filaments joined by continuous organic union — the growth 

 of connective tissue and epithelium having, in these interfila- 

 mentar spaces, replaced or supplemented the usual ciliated 

 junctions. Thus the two forms of junction exist side by side or 

 form a compound union, and all degrees of formation of a com- 

 plete cellular connection may be found (see fig. 12). It is 

 scarcely possible, after examining such conditions, to doubt that 

 ciliated junctions have preceded the organic union. Pigure 11 

 shows two filaments joined by the compound of ciliated disc and 

 cellular connection, and fig. 12 shows the presence together in 

 the one plica, at the same level, of ordinary ciliated junctions 

 and various degrees of organic growth. 8ince these figures were 

 drawn I hnve found a specimen where all the twelve filaments of 

 a plica were united by complete organic union (Pi. 27. figs. 7 c 

 and d). The concrescence is not always at the internal edges of 

 the filaments, but may be about the middle of the former ciliated 

 disc ; and in one case I found two points of union between two 

 neighbouring filaments leaving an ovate ciliated gap. 



