PHOLAS. 189 



action never ceases whilst moisture remains in the shells, and 

 I think it must be considered as settled, that there is no com- 

 munity between the cilia and what are called branchial currents. 

 I have at Exmouth repeated all the experiments with the 

 mercury on fresh flexible animals ; the first were performed 

 with rigid specimens from spirit ; the results are most satis- 

 factory, and I think entitle me to state with confidence, that 

 in Pholas there is no communication between the branchial 

 and anal siphons. 



Since the above observations were written, I am enabled 

 to state, after a prolonged and anxious examination of fifty 

 living Pholades, under all the phases of experiment, that nine- 

 tenths, if not all the water to bathe the branchiae is admitted 

 at the pedal gape, and ejected only by the branchial siphon; 

 the anal one alone inhales water and discharges it; and in 

 the closed-mantle Solenidae, Myada, Lutrarice, &c., as well as 

 in the open-mantle Veneres, Cardia, &c., the water is only 

 admitted into the branchial vault at the pedal or ventral 

 aperture by the simple opening of the valves, and ejected 

 according to the structure of their respective sacs, either by 

 the branchial issue alone, as in the Pholades, &c., or, as in the 

 Veneres, Cardia, &c., by the two confluent orifices, which are 

 in fact but one branchial conduit. 



This discovery and attendant results will finally, I hope, 

 dispose of the complicated scheme of some authors, of the 

 reception and discharge of the water for branchial purposes 

 by cilia and separate siphonal ducts, as it shows what I have 

 always advocated, that nature gives access to the water for the 

 respiratory apparatus by the simple opening of the valves, and 

 causes it to be discharged, when effete, by their closure at the 

 posterior siphonal issue, as well as by the pedal opening and 

 ventral scissions of the mantle. It is therefore I think satis- 

 factorily proved, that the doctrine of separate currents by cilia, 

 and that the inhalant is always kept distinct from the exhalant 

 current and admitted by a separate aperture from that by 

 which the latter is expelled, or in other words, that the water 

 is imbibed by the branchial siphon and discharged from the 

 anal, is absolutely untenable. 



