188 PHOLADID A. 
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., 
wherei the branchial sac is posteriorly divided into two, not 
separated, but confluent siphons at their bases, bemg 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 im 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 similar category as to the mode of admission 
of the water to the branchiz, whatever may be the differences 
in certain classes in the disposition and structure of the 
siphonal apparatus. 
Haying 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 
