202 PHOLADIDJE. 



municate through minute openings in the laminae of the 

 gill-plates. 



" Having thus satisfied ourselves of this fact, we next directed 

 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 orifices,, already well known to anatomists, correspond 

 to the attached 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 com- 

 municate freely with the anal chamber. The laminae forming 

 the walls of these tubes were now examined through the mi- 

 croscope, 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 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 de- 

 sired end. 



f ' 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 

 counter-statement. If these gentlemen had read a little more 

 attentively, they would have seen, in the paper on which they 

 have passed their strictures, that Mr. Clark states, " The appli- 

 cation of the mercury to that tube gradually filled the entire 

 range of the branchial vessels, which exhibited a very elegant 



