ANATOMY' OF THE I'KAKL OYSTER. 61 



.Hid more highly magnified, is shown in fig. 14. The epithelium on the surface is 

 ectoderm, and this varies in height in certain parts, and is ciliated along special tracts, 

 the chief of which are the frontal and the lateral (Plate VIII., figs. 13, 14). Under- 

 neath the epithelium of the gill filament is a thin layer of connective-tissue 

 strengthened by chitinous thickenings. These skeletal thickenings take on a special 

 development in the principal filament lying in the angle hetween two ridges 

 (Plate VIII., fig. 13). Connective-tissue septa do not occur in the interior of the 

 ordinary filaments, but at the level of the ciliated junctions the modified filaments 

 which bear the discs may have their lumen largely obliterated by an unusual develop- 

 ment of connective-tissue (fig. 15). Further details can be seen in the figures. 



Apart from the special action of modified cilia in forming an interlocking junction, 

 the normal function of the ordinary cilia on the branchiae is to create the all-important 

 current of water which enters the pallial chamber and passes over and through the 

 branchial lamellae, so as (1) to aerate the blood flowing in the filaments, aird (2) to 

 convey food particles to the mouth. The respiratory current is apparently due to the 

 normal rhythmic lashing of the cilia on the large cells at the edges of the filaments ; 

 while the collection or rejection of particles in the water seems to be the result oi 

 special action stimulated apparently by the irritation. Particles arrested by the 

 branchial filter are caught up by the nearest cilia, which by local reversed lashing carry 

 them outwards to the free ventral edge of the lamella. Here they are guided by the 

 cilia of a pathway running along the branchial margin and are propelled forwards and 

 upwards to the anterior end of the gill, where they come under the influence of the 

 palps, to be accepted as food or rejected and conveyed to the exterior by the pallial 

 ciliated band. On Plate VI., fig. 13 shows the ciliated paths, upon and between the 

 gills, by which particles can approach the palpal gutters (6.) leading to the mouth (.) ; 

 and fig. 14 shows the track (Pall.cil.b.) along the mantle edge by which excreta pass 

 to the exterior at the pallial fold (Pall./.). 



THE VASCULAE SYSTEM. 



In common with all tvpical Lamellibranchs, Margaritifera vulgaris has a 

 circulatory system consisting of a heart and a series of arteries, whence by means of 

 irregular ill-defined spaces, the lacunas, between and among the tissues and organs 

 the blood flows into larger and usually well-defined thin-walled cavities, the venous 

 sinuses. The bulk of the blood then circulates through the gills prior to being- 

 returned to the heart, but a portion passes direct from certain sinuses in the mantle. 

 Of these vessels the arteries alone have definitely cellular walls, those of the sinuses 

 being of connective-tissue. 



The heart is contained in a thin-walled transparent sac, the pericardium {Per., 

 Plate VII., fig. 2), occupying nearly all of the posterior region of the body, the space 

 bounded in front by the posterior limit of the viscero-pedal mass and behind by the 



