198 PHOLADIDJE. 



from the consideration that the cilia must act antagonistically 

 from opposite points; one set to work the water in bran- 

 chially, and another to expel it through the anal duct after 

 percolating the gill-laminae and interbranchial tubes. I shall 

 recur to the cilia, and expect to prove that the new scheme of 

 communication between the two siphons is very problematical, 

 I may say impracticable. 



Messrs. Alder and Hancock go on to say, that any one 

 may convince himself of the existence of a branchial in-current 

 and an anal ex-current, by placing a Pholas " in a glass of 

 sea- water, and then by gently adding a little fresh, slightly 

 charged with floating particles," the two actions may be per- 

 ceived. I admit, by this process, that currents will ensue, as 

 the invigorating fresh element causes the animal to expel that 

 which has become effete and take in a new supply ; but as I 

 have, under every condition of experiment, examined multi- 

 tudes of these creatures, I am bound to declare that the 

 currents have no continuous regularity : and I think the mode 

 of testing their presence by means of water charged with 

 buoyant particles is fallacious ; these only float on the sur- 

 face, and are subject to many perturbations and deceptions 

 arising from depth of water, currents of air, the position of 

 the animal, whether on the ventral or dorsal surface, by its 

 will and humour, state of exhaustion, and an unnatural con- 

 finement. My repeated examinations show that the parti- 

 cles are whirled in all directions ; sometimes they pass into 

 the branchial chamber, at others none will enter : capricious 

 gyrations, whether arising from the action of the animal or 

 other natural causes, are their ruling character. With respect 

 to the anal siphon, the floating particles are certainly repelled 

 from its orifice in a somewhat regular and continuous manner, 

 being only interrupted by the periodic reception of fluid to 

 supply the exhaustions. The frequent repulsions of particles 

 from the anal orifice have been construed by Messrs. Alder 

 and Hancock to arise from the percolation of water from the 

 branchial vault through the gill-laminae and interbranchial 

 tubes to an issue at the anal siphon, produced by the agency 

 of cilia. I think it will appear that this complicated operation 



