312 Mr. W. Clark on the Branchial Currents in the Bivalves. 



fluids, the invariable result lias been rny inability, as in the 

 first experiments in 1850, to pass the fluids through the anal 

 chamber further than to All all the interbranchial tubes ; but I 

 always found the gill-laminae, which form their walls, impervious, 

 instead of allowing liquid to issue " from 10,000 pores." It is 

 necessary to state that the numerous interlaminar canals that 

 compose the divisions of the gill-plates are nearly parallel, and 

 hang vertically from the dorsal line, ranging at equidistances 

 throughout a great part of the extent of each branchial, plate, 

 and by sutural lines of junction cut ofi" the communication be- 

 tween each tube. 



I will now enter a little more into detail on some points in 

 connexion with the branchial laminse, by describing the appear- 

 ance of the areas of the parallelograms under repeated examina- 

 tions by transmitted light, and also as opake objects, rendered 

 so by the injection of mercury. 



In a full-grown Pholas dactylus, the surfaces of each gill- 

 lamina together comprise an extent of about a square inch, 

 every one-tenth of which contains 400 oblong subquadrangular 

 spaces, or 40,000 in each plate, forming a total in the four gills 

 of 160,000; this admeasurement and enumeration may not be 

 very far from the truth. In each parallelogram, besides a ge- 

 neral suboval depression, there are within it five to twenty or 

 moi-e shallow excavations of various size and shape, but there is 

 no ruling symmetrical fissure as delineated in Messrs. Alder and 

 Hancock^s fig. 3. Each area shows a plain, a pitted, and a mam- 

 millated or traceried surface, detected by the action of the micro- 

 scopic foci. We will start from the plain surface in which there 

 is certainly no perforation ; the fine adjustment of the instrument 

 measures the depth of the depressions, and by another movement 

 shows the character of the minute points, thus proving that no 

 fissure or aperture exists, as when there is really an imperfection 

 in the membrane it cannot thus be resolved, but under every phase 

 of the instrument the hiatus of a solution of continuity is seen. 

 The shallow depressions are the uncovered patches of the mem- 

 branous base of the scales or epithelium incident to all the MoL- 

 lusca ; from them the numerous vibratile cilia spring which pre- 

 sent the most discordant and particular motions that operate 

 from every point ; sometimes they appear as if each entire pit 

 was whirled on a vertical axis, at others a compact mass of 

 strands dilates and contracts like the heart, then a fasciculus of 

 cilia is seen beating the water with every irregularity ; sometimes 

 only a single cirrhus is raised and falls in quick succession, like a 

 hammer in a mechanic's hand ; but it is impossible to describe all 

 the varieties of motion. In a fresh animal the action and strokes 

 exhibit the greatest rapidity ; it seems utterly impracticable that 



