310 Mr. W. Clark on the Branchial Currents in the Bivdlve^i 



" But a simple experiment will at once solve this difficuHyi'^ 

 Having killed a specimen of Pholas crispata with the siphonal' 

 tubes contracted as little as possible, and having placed it in di- 

 luted spirit a few hours to render the tissues firm without hai-d- 

 cning them too much, we had again recourse to the blowpipe, 

 charged as formerly with coloured fluid. The specimen was 

 opened down the venti'al margin, exposing to view the whole of 

 the gills stretched along the roof of the branchial cavity. The 

 nosle of the blowpipe was passed into the anal siphon, and on 

 removing the finger from the top of the pipe, the contained fluid 

 immediately filled the anal chamber behind the gills, and then 

 passing at once down the tubes between the laminse of the gills, 

 issued through ten thousand pores, and dyed the water in the 

 branchial chamber. Thus in an instant the secret was ex- 

 plained ; — the currents communicate through minute openings 

 in the laminre of the gill-plates. '' '' 



" Having thus satisfied ourselves of this fact, we next directeid ' 

 our attention to the structure of the gills. Accordingly the anal 

 chamber was laid open, and its ventral wall was seen to exhibit 

 four longitudinal rows of large orifices. These four rows of ori- 

 fices, already well knowoi to anatomists, correspond to the at- 

 tached margins of the four gill-plates, which hang from the roof 

 or dorsal membrane of the branchial chamber ; this membrane 

 being the ventral wall of the anal chamber, — the membrane, in 

 fact, which divides the chambers. 



" These orifices lead into wide tubes which pass between the 

 two laminae forming each gill-plate. These interbranchial tubes 

 lie contiguous and parallel to each other, and extend the full 

 width of the gill, being bifid within its free margin. Thus it is 

 evident that the tubes within the gill-plates communicate freely 

 with the anal chamber. The laminse forming the walls of these 

 tubes were now examined through the microscope, when the 

 whole was observed to present a regularly reticulated stiiicture 

 composed of blood-vessels ; those passing transversely being the 

 stronger and more prominent. The longitudinal vessels, rather 

 far apart from each other, form the meshes into parallelograms. 

 These meshes are open spaces, fringed internally with a narrow 

 membrane and active vibratile cilia. The two vascular laminae 

 forming the gill-plate are really sieves to separate suspended 

 molecules from the surrounding medium on the passage of the 

 water from the branchial to the anal chamber ; — an apparatus of 

 the most exquisite beauty and perfect adaptation to the desired 

 end. 



"We cannot understand how this beautiful structure escaped 

 detection by the mercurial injection of Mr. Clark." 



I at once dispose of the last remark to save trouble in my 



