188 PHOLADID^E. 



is clear that regular separate currents by cilia cannot exist 

 between the two siphons, so as to make one the inhalant, 

 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 Veneres, 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 conjunc- 

 tion, 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 ' f ex uno disce omnes," to consider 

 that all are in a similar 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 crevices 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 con- 

 dition 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, sub- 

 dividing, 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 produced 

 from nothing. 



It appears then, whether the cilia be within the possibility 

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



