ACEPHALA LAMELLIBRANCHIATA. 25 
venous blood enter by branches into the principal branchial 
vein and auricles, and is sent into the circulation without 
being transmitted to the branchiz for aération. Part of these 
views I can confirm, as far as relates to the spongy excretory 
glands at the anterior end of the sinus above alluded to; they 
are visible in Pholas dactylus, and conspicuously in P. parva ; 
their uses are fully described under the title of excretory 
organs of P. dactylus. As to a portion of the blood being 
returned to the heart without receiving the dose of oxygen, I 
have not had it in my power to verify that fact. 
From this statement it would appear, that i some of the 
bivalves, if not in all, the circulation is not without imterrup- 
tion, and that there is a particular one for the special purpose 
of providing the necessary secretions. With regard to these 
poimts Iam inclined to think, that the vems of each organ 
form and supply the secreting glands for itself, in its own 
immediate vicinity; for instance, the liver, the bile for the 
stomach ; the ovarium, the mucus for the ova; the byssal 
gland, the filamentous fluid; the mantle its own pigments ; 
also that thin membrane which is the external duplicature on 
the shell, springing from its margins, and spreading more or 
less on all shells, as well as on some of the exposed soft parts 
of the animal, producing what is termed the pilose skin, epi- 
dermis, and exterior ligamental covering. There is a difficulty 
in supposing that all the secretions are elaborated and distilled 
from a mass of veins collected m a particular cavity, and pro- 
duce the peculiar secretion for each organ, conveying it thereto 
by a separate sac or distinct duct. I can scarcely believe that 
such a concentrated olla podrida can act harmoniously, and 
produce simultaneously, from a fasciculus of vems deposited in 
the same reservoir, mucus, pigments, carbonate of lime, &e. 
With respect to the digestive organs, the mouth — there 
bemg no head, eyes, nor tentacula—is a large, plain, trans- 
versely oval aperture, covered by the mantle, and is placed 
towards the upper part of the anterior range, having on each 
side of it a pair of palpi, which appear to be both of a tenta- 
cular and respiratory nature, and perhaps assist the cilia that 
clothe the branchial laminz to beat and divide the water in 
