220 PHOLADIDZ. 
a pulp, which under the microscope has the aspect of ligneous 
debris. The Patelle operate in like manner. 
The circulation is venous, arterial and branchial, and con- 
sequently complete. The respiratory apparatus has been 
strangely misunderstood ; it has been described to consist of 
four fleshy cords, portions of which Sir Everard Home pro- 
nounced to be the testes, and others the ovaria; these views 
are erroneous. But we will first mention the heart and auri- 
cles, which are placed at the base of the ovarium in the peri- 
toneal cavity within the mantle, but in a distinct pericardium ; 
the heart is an elongated, very pale bluish-white opake ven- 
tricle, accompanied by two symmetrical fusiform slender auri- 
cles that are also opake, somewhat posterior to it, which 
appear to pour the aérated blood imto it by lateral valvular 
ducts. On opening the ventricle its walls did not exhibit any 
particular muscularity: we were not successful in detecting 
the valves of the auricles. There is at the posterior part of 
the auricles a white, suboval, subglobular, fine granular mass, 
touching and partly surrounding them; we are unable to 
state its nature; it is not part of the ovarium, which termi- 
nates before the pericardium commences, and in such a situa- 
tion it cannot be the organ to animalize the ova: we are in- 
clined to consider it a gland that distils a liquor for the use 
of the heart and auricles. 
At the base of these organs the four cords that have created 
such difference of opmion as to their uses come into view, but 
they do not appear to be either the branchie, arteries, veins, 
testes or ovaria; still they have a sort of connection with the 
branchiz ; the two longer and larger brown lines have their 
origin on each side the hemispherical valves, and proceed, 
attached to each latero-dorsal range of the mantle, to the 
posterior siphons ; they appear to be composed of red-brown 
granular points; within these two lines, but not until the 
branchiz commence, two others of smaller size and nearly 
similar composition run parallel, and terminate with the larger 
ones at the siphons; the addition of the two shorter and 
smaller cords springing from the larger at the pomt where 
the branchiz begin, appears to show a connection of these 
