ON MOLLUSCA. 915 
The oysters have also the heart placed differently, and not 
occupying the back of the animal, but the anterior part of the 
central muscle. 
In the naked acephala it occupies pretty nearly the same 
place as in the conchiferous, but it is perhaps less symme- 
trical. 
It is completely so in the nematopods, and especially in the 
oscabriones, whose ventricle occupies the posterior part of 
the back, along with a large auricle, very symmetrical on each 
side. 
We shall now speak briefly of the urinary and generative 
processes. 
The apparatus of urinary depuration, in general very simple, 
appears to exist in all the malacozoaria, that have been suffi- 
ciently examined. It always accompanies the termination of 
the intestinal canal. In the cephala it is found sometimes 
described under the names of the organ of glue, or the cal- 
careous sac, and in the acephala under that of pulmonary 
organ. 
In the first it consists of a single secretory organ, not sym- 
metrical, of very variable form, often situated near the organ 
of respiration, making a projection into the interior of the 
cavity which contains it. From it springs an excretory canal, 
which, after a passage more or less long, often accompanying 
the rectum, terminates externally by a rounded sessile orifice, 
at a small distance from the anus. 
In the class of the acephala the depurator organ is double, 
or at least symmetrical. It is situated equally on each side 
of the rectum underneath it, in front of the posterior adductor 
muscle, and behind the connexion of the branchial lobes. Its 
colour is usually of a deep green. Its form is more or less 
cylindrical, its structure is cellular or vascular, and it receives 
a great number of arterial, and more especially of venous 
vessels. It appears that the excretory canal is not sufficiently 
