ANATOMY OF THE PEAEL OYSTER. 59 



downwards, it follows that these branchial filaments must be directed in the upper 

 or oral regioi) forwards, and in the lower or ventral downwards in all cases towards 

 the nearest margin of the valve. 



The outwardly directed parallel filaments of each series are folded upon themselves, 

 so that they become deeply V-shaped. The folding in each case is away from the 

 common centre or axis, the external filament turning outwards and the internal 

 inwards. Consecpiently, each branchial plate, formed of the doubled filaments, 

 consists of two lamellae the direct and the reflected which enclose between them a 

 narrow inter-lamellar space (see Plate V., fig. 4) where the outer lamella of the 

 external hemi-ctenidium is the reflected, and the inner (that which is attached to the 

 vascular axis) is the direct the converse being the case with the internal hemi- 

 ctenidium where the outer lamella is the direct one : the two direct lamellae face one 

 another. 



Immediately behind the visceral mass the edges of the reflected lamella3 of the 

 inner gills (a.g.f., Plate VIII., fig. 7) of the two sides join loosely in the middle line, 

 so as to show in a transverse section the form of two capital W's, imperfectly joined, 

 thus WW (Plate V., fig. 9, Br.). The free outer edge also adheres somewhat, 

 under normal conditions, to the adjacent mantle by a thickened rim ; but both the 

 median and the lateral concrescing surfaces are readily separated with a little 

 pressure, and it can scarcely be said that there is permanent fusion. 



Histological examination of these places shows that the union is by means of very 

 long and perfect ciliary junctions (Plate VIII., figs. 8, 9, 10), closely resembling in the 

 appearance both of cells and cilia the ciliated discs of the filaments. At the extreme 

 ventral edge of the median junction of the inner gills there is a very narrow but quite 

 definite organic connection (Plate VIII., fig. 9, org.) which must be ruptured when 

 the gills are pressed apart. At these ciliated junctions the epithelial cells are cubical 

 or low columnar, with a distinct seam or margin from which the very regular stiff 

 cilia project (figs. 9 and 10). 



The common base (ct.a., Plate VIII., fig. 6) of each ctenidium is a vascular 

 attached ridge reaching from the anterior end of the gills, overlapped slightly by the 

 bases of the palps, to a point near the anterior end of the adductor, thence running 

 as a free axis to the posterior or distal extremity. It is seen in both conditions in 

 sections on Plate V., attached in fig. 4 and free in tig. 9. Within the axis lie two 

 great blood-vessels, the afferent and the efferent branchials. The former (Br.aff., 

 Plate VIII., fig. 8), which conducts the blood from the venous sinuses to the gills for 

 purification, lies internal and dorsal to the latter (in Plate VIII., fig. 8, it appears as 

 a tube above the efferent vessel, Br.eff.). 



Hollow outgrowths, the inter-lamellar junctions, containing branches from the 

 afferent vessels, convey blood from the axial trunk to the base of the reflected 

 lamella. Thence the blood enters certain of the individual filaments, flowing out- 

 wards to the free margin, where it passes over into the direct filaments and so 



I 2 



