Currents in Pholas and Mya. 375 



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 laminae forming the walls of these tubes 

 were now examined through the microscope, when the whole was 

 observed to present a regularly reticulated structure 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 mem- 

 brane and active vibratile cilia. The two vascular laminae form- 

 ing the gill-plate are really sieves to separate suspended mole- 

 cules 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. Had the anal 

 chamber been completely filled with mercury as supposed, we 

 conceive that it must necessarily have found its way into the in- 

 terbranchial tubes, and probably thence into the ventral cavity. 

 Indeed we think that so large a quantity of mercury as was re- 

 quired to fill the anal cavity would have caused a rupture of the 

 very delicate membrane dividing this chamber from the branchial 

 one, even without the external pressure which Mr. Clark says he 

 applied to it. The interbranchial tubes have been described by 

 several anatomists, but they do not seem to have carried their 

 examinations so far as to have ascertained the subserviency of 

 these tubes to the branchial currents. They have in a few genera 

 been described to be for the purpose of receiving the immature 

 ova after leaving the ovary. 



Having carried our inquiries so far, we were anxious to ob- 

 serve this exquisitely constructed apparatus in action, and accord- 

 ingly laid open the branchial chamber of a living animal, by di- 

 viding the mantle along the ventral margin, and thus completely 

 exposing to view the gills. The specimen was then placed in a 

 shallow vessel containing sea-water, and in a short time the ex- 

 current siphon was in full play, pouring out a constant, steady 

 current. As the branchial siphon was laid open from end to end, 

 its action was stopped ; the branchial chamber, however, being 

 submerged was always filled with water. We now slightly 

 coloured the fluid with indigo and carefully watched the gills 

 through a rather powerful lens, which revealed a sight of extra- 

 ordinary interest. The gills lay stretched, two along each side 

 of the body, their posterior extremities passing some way up the 

 branchial siphon, and appearing like delicate waved fringes, with 

 their surfaces transversely fluted ; the flutes being caused by the 



