374 Messrs. Alder and Hancock on the Branchial 



convulsive action, squirted out the coloured matter by the same 

 orifice by which it was received. It is worthy of remark, that, if 

 we did not succeed in the first attempt, the second was almost 

 sure to fail ; the internal apparatus being apparently then ad- 

 justed to prevent the passage of the coloured particles : and it is 

 likewise remarkable that the coloured fluid exhaled was deprived 

 of all the coarser particles. 



These experiments may be deemed conclusive of the fact that 

 the inhalant and exhalant currents are really connected. The 

 mode of their connexion, however, remains up to this time undis- 

 covered. For the purpose of explaining this difficult point, we 

 have found it necessary to make a careful examination of the 

 anatomical structure of these animals. The result is very satis- 

 factory. We certainly find no opening between the foot and the 

 gills, nor between the gills and the mantle. The branchial or 

 ventral chamber is so far completely isolated from the anal or 

 dorsal chamber; and without a careful observation of the cur- 

 rents in the living animal, as above alluded to, here the matter 

 might have been allowed to rest. 



But a simple experiment will at once solve this difficulty. 

 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 hard- 

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

 charged as formerly with coloured fluid. The specimen on this 

 occasion was opened down the ventral 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 explained ; — the currents communicate through mi- 

 nute openings in the laminse 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 ori- 

 fices, already well known 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 

 laminse forming each gill-plate. These interbranchial tubes lie 



