202 PHOLADIDA. 
municate through minute openings in the lamine 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 laminz forming each gill-plate. These imterbranchial 
tubes lie contiguous and parallel to each other, and extend 
the full width of the gill, bemg bifid within its free margin. 
Thus it is evident that. the tubes withm the gill-plates com- 
municate freely with the anal chamber. The laminz 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 promiment. The 
longitudinal vessels, rather far apart from each other, form 
the meshes into parallelograms. These meshes are open 
spaces, frmged internally with a narrow membrane and active 
vibratile cilia. The two vascular lamine 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. 
“ 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 
