Mr. W. Clark on the Pholadidfc. 325 



and anxious examination of fifty of the 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 gaTpe, and ejected 07ily 

 by the branchial siphon; the anal one alone inhales water and dis- 

 charges it; and in the close- mantle Solenidce, Myadce, 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 accord- 

 ing to the structure of their respective sacs, either by the branchial 

 issue alone, as in the Pholades, &c., or as in the Veneres, Car- 

 dia, &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 sepa- 

 rate 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 satisfactorily 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. 



The important discovery I have just related was made mani- 

 fest in the simplest manner. On taking up an animal, the siphons 

 of which were largely inflated, I observed that the great mass of 

 water was poured out from the branchial tube, and only a small 

 quantity from the anal one ; on replacing the animal in water, I 

 was surprised, instead of seeing, as I expected, the water flow up 

 the branchial canal, to observe a powerful column, through the 

 tenuity of the membrane, pass rapidly from the pedal open- 

 ing, in consequence of the relaxation of the mantle around the 

 pedal gape, and fill the branchial vault. This very decisive proof 

 how the water reaches the branchiae induced me to vary the ex- 

 periment. I placed the animal with the tubes entirely in the 

 water and the pedal gape out of it ; very little fluid entered the 

 branchial sac, the anal siphon alone imbibed a portion ; and 

 on holding the animal with the siphons downward, scai'cely any 

 water issued from the branchial one, and only a little from the 

 anal ; but as soon as I suffered the pedal gape to reach the water, 

 a column was instantly seen to fill it as before. 



I do not mean to say that if the pedal aperture is kept out of 

 the water, some fluid may not be imbibed by the branchial 



