323 Mr. W. Clark on the Plioladidte. 



branous narrow symmetrical laminae on each side of the body^ 

 composed of a vascular network, fixed under the mantle to the 

 dorsal range, accompanied also on each side by a pair of palpi : 

 on leaving the body the four branchiae without an intermediate 

 substance run together tapering to their termination at about 

 half an inch from the extremity of the branchial orifice ; they are 

 firmly fixed the whole length by their bases from the point they 

 leave the body to the membrane which separates the anal from 

 the branchial cavity, cutting off all communication between the 

 two siphons ; consequently the water must be received and ejected 

 through the branchial siphon, or by the pedal orifice, in the bi- 

 valves with closed mantles, as is the case with the Pholades, and 

 probably with other families of the same structure in which the 

 branchial and anal tubes are separated their entire length by a 

 divisional membrane. 



The discovery of this circumstance is so important as regards 

 the disputed point, how the branchiae receive the ambient element, 

 that I have used every means to test it, by dissection, by all the 

 modifications of experiment, and particularly by mercurial injec- 

 tions ; this last mode I have used very successfully, as the follow- 

 ing operations will show. First, I threw into the anal siphon a 

 column of mercury that completely filled the cavity, and on ap- 

 plying further pressure regurgitation ensued, but not a particle 

 of the mineral found a passage to the branchial vault ; this result 

 occurred in many specimens, and though the pressure was often 

 considerable before regurgitation was allowed to take place, still 

 the branchial division of the mantle remained free from the quick- 

 silver. In one experiment the fluid appeared in the lateral tis- 

 sues ; this condition I attribute to a lesion of the dorsal lateral 

 membranes. 2ndly, I found that if there is the slightest solution 

 of continuity in the tine membrane on which the branchial ves- 

 sels are fixed on one side, and the interweavings and traceries on 

 the other, which form the roof of the anal siphon, the application 

 of the mercury to that tube gradually filled the whole range of 

 the branchial vessels, which exhibited a very elegant appearance, 

 but no fluid escaped from them into the branchial sac. 3rdly, I 

 repeated many times the first experiment with the view of endea- 

 vouring to find a passage through the rectum, and intestine on 

 the dorsal range that is embraced by the heart, but without suc- 

 cess ; if I had succeeded, it would have been impossible to arrive 

 at the stomach and mouth, as the intestine plunges into the body 

 coasting the foot, at which point it is of larger diameter, and 

 always filled with a compacted mass of sand which effectually 

 stops up the passage : this part of the intestine around the foot, 

 from its difficulty and the hardness of the faecal matters, seems 

 analogous to the ascending portion of the colon in man. 



