324 Mr. W. Clark on the Pholadidse. 



and the other the exhalant canal ; and if there is any truth in 

 analogy, every presumption authorizes us to conclude, that the 

 same action of the reception and discharge of the water through 

 the branchial siphon and pedal or ventral opening prevails in 

 the open-mantle bivalves, the Venei-es, Cardia, &c., wherein the 

 branchial sac is posteriorly divided into two, not separated, but 

 confluent siphons at their bases, being only more or less divided 

 towards their terminal portions by an internal septum, so that 

 they must be considered in conjunction, and as one siphon, for 

 branchial purposes. 



If therefore it be established beyond all reasonable doubt, that 

 there is no communication between the anal and branchial siphons 

 in Pholas, there is an end to the doctrine of separate branchial 

 currents by cilia; for if this is impossible in one family of the 

 same class, we have a right analogically, and agreeably to the 

 axiom, " ex uno disce omnes," to consider that all are in a simi- 

 lar category as to the mode of admission of the water to the 

 branchiae, whatever may be the differences in certain classes in 

 the disposition and structure of the siphonal apparatus. 



Having arrived at this conclusion, I will, though it is almost 

 unnecessary in corroboration of it, make a few additional remarks. 

 It is well known that muscles are often hung up high in the cre- 

 vices of rocks, some of them above the level of the ordinary tides, 

 where my dredger says that they remain suspended throughout 

 the year, and can only for a few days in each month, at spring 

 tides, receive the water : this condition may occur for about two 

 hours in 75 days out of the 365 ; yet when any of these animals 

 are opened, the cilia under the microscope will always be seen in 

 action, beating, subdividing, and eliminating the air from the 

 moisture. In this case, for near three-quarters of the year, the 

 creation of branchial currents is impossible ; they cannot be pro- 

 duced from nothing. 



It appears then, whether the cilia be within the possibility of 

 assisting in the creation of branchial currents or not, their action 

 never ceases whilst moisture remains in the shells, and I think 

 it must be considered as settled, that there is no community 

 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 speci- 

 mens from spirit ; the results are most satisfactory, and I think 

 entitle me to state with confidence, that in Pholas there is no 

 communication between the branchial and anal siphons. 



I have now to offer a most important communication, which I 

 have only been in a condition to make since the above observa- 

 tions were written. I am enabled to state, after a prolonged 



