60 CEYLON PEARL OYSTER REPORT. 



returns inwards to the branchial axis, where it joins the efferent vessel by openings 

 along each side. The branchial afferent vessels and the band-shaped inter-lamellar 

 junctions (Plate VIII., figs. 8 and 12) are comparatively few, and each serves a group 

 of 10 or 12 of the reflected filaments. On the other hand, each direct filament has 

 its own aperture into the efferent vessel. 



The filaments composing a lamella are not placed in one plane. On the contrary, 

 the lamella is pleated or plicated regularly at right angles to its base, so as to 

 have alternating shallow channels and rounded ridges (Plate VIII. , fig. 12). The 

 number of filaments constituting a plica varies from 10 to 12. The transverse section 

 of the gills given in fig. 12 on Plate VIII. shows how the ridges are formed by the 

 plication of the filaments. At the bottom of each channel there is a sj:>ecially large 

 and modified filament (Plate VIII., figs. 12 and 13, p.f.) with a great development of 

 skeletal chitin and some muscle bundles. These are known as the principal filaments, 

 and they have the inter-lamellar junctions attached to them and alone receive afferent 

 branchial vessels. The aerated blood passes from the gills by the ordinary filaments. 



Neighbouring filaments are joined by continuous organic union mainly at the lower 

 and the upper ends of the reflected filaments, where there are longitudinally-running 

 blood vessels. Elsewhere the filaments are joined chiefly by the interlocking stitt 

 cilia of the large ciliated discs which occur at intervals (Plate VIII., fig. 11, c.d.) 

 throughout their length. In many places, however, groups of two or three or more 

 filaments (see figs. 16, 17) are united by true organic junctions which occur alongside 

 the ciliated discs, as Rjdewood* suggested might possibly be the case. Fig. 16 

 shows four filaments united, I have found several examples of six, and in one specimen 

 the whole twelve filaments of a plica were joined by continuous tissue. The con- 

 crescence is not always at the internal edge of the filaments, but may be about the 

 middle, and in one case I found two unions between two neighbouring filaments 

 leaving an ovate ciliated gap. But all such examples of true organic union are com- 

 paratively few and exceptional, and we certainly do not have in this gill the continuous 

 solid inter-filamentar junctions which are found in the less simple gills of the 

 Eulamellibranchiata (such as Venus, Cardium, Mya and Anodonta). 



The frequency and arrangement of the ciliated discs is seen in fig. 11, representing 

 a longitudinal section along several adjacent filaments, and a transverse section at 

 the level of these junctions is seen in fig. 16. The epithelial cells bearing these 

 special stiff cilia project beyond the general surface, and are of cubical or low 

 columnar form. 



The gill of the pearl oyster is thus what Pelseneer termed " Pseudolamellibranch," 

 the lamellae being plicated and connected by inter-lamellar vascular junctions, while 

 the individual filaments are mainly united by the interlocking of the ciliated discs. 



In transverse section the filament has a bluntly wedge-shaped outline, the narrower 

 end being internal. The structure of an ordinary filament, where free from junctions, 



* ' Phil. Trans.,' vol. 195, B, p. 155. 



