Am) OYIDTJCAL SYSTEM IN THE LAMELLIBEANCHIATE MOLLHSKS. 
31 
divergent twigs of the principal branches of the water-vascular tree ; they do not lie in 
the direct course of the current of the aquiferous canals, and these canals, beyond and 
outside of them, break up into a very delicate minutely divided system of capillary tubes. 
What we shall attempt to prove is, that the orifices on either side of the foot in the 
Unionidse lead not only to the generative gland, the products of which may be seen to 
issue forth from them at the spawning-season, but also to a system of tubes widely 
spread through the entire foot. We do not believe that any direct communication sub- 
sists either between the blood-vascular system and this system of tubes, or between either 
of these systems and the punctated depressions and inlets along the foot-edge. The 
blood-vessels seem to us to constitute a system of tubes closed, save at one point and at 
one lacuna. That point and that lacuna is the pericardial space — a cavity into which, 
besides the blood of the animal, the water in which it lives also finds its way. As the 
bivalve sheU opens, it necessarily dilates this lacuna, and water is thus drawn into it 
through the compound sac known in the Acephala as the organ of Bojanus. The water 
then gains access to the interior of the blood-vessels, as we shall proceed to show, and 
is carried onward within them. From the blood-vessels we suppose it to transude into the 
system of water-tubes eveiywhere in apposition with them, and, under normal conditions, 
to find its exit by these tubes, whilst under such abnormal circumstances as the sudden 
removal of the creature from the water, the sudden contraction of the muscular foot, 
causing jets of water to pour forth from the dilated semitransparent mass, may unload the 
infiltrated organ in a yet more expeditious mamier. As to the way by which the water 
used by the mollusk for distending its foot comes into the body, we are at one with 
many other writers upon this subject; but we are not aAvare that our views, as to the 
method by which the animal disencumbers itself of the ingested fluid, are shared in by 
other authors. 
Our arguments will be principally based upon the results of experiments made in the 
way of injection. The animals we operated upon were almost exclusively of the family 
Unionidae ; and, on account of the size of the specimens, as well as for other reasons, we 
employed chiefly the species Anodon cygneus and Unio margaritifera. In all our expe- 
riments we strove to reproduce, as nearly as possible, the conditions of the animal’s natu- 
ral life : our injections were always performed under water, by which and by other 
means as much support was given to the animal’s body and its several parts as the water 
and the shell gave to it during life. Means were always adopted for securing that the 
animal died with its muscular system in a state of relaxation. We found the prussian 
blue injecting-fluid of Professor Beale’s* invention to possess many properties espe- 
cially recommending it for use in our experiments, but we employed several other fluids 
as well. 
Experiment 1. — If an Anodon or Unio (size is of little consequence in this experi- 
ment, though large size is a convenience in most) be removed from its shell without 
injuring the somewhat easily injured tissues which limit the secreting-structure of the 
* How to work with tke Microscope, p. 78, 1857. 
