ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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only with the pericardium; the right organ, which has alone the 
depuratory function, has lost this communication. This right kidney 
is placed in the course of the blood which comes from the lacunae of the 
body to pass to the gills. In Haliotis all the blood traverses it ; in the 
Trochidae part only ; in the Monotocardia it receives but a small part 
of the blood, and a special vascular area is formed for it with afferent 
and efferent passages which are independent of the general circula- 
tion. The left kidney of the Diotocardia, as well as that of Patella and 
the Heteronephridiata, has a vascular system directly connected with 
the auricle or auricles. The Neritidee have only one kidney. 
The Monotocardia have a single renal cavity and a single excretory 
orifice. The latter is simple and is placed at the base of the pallial 
chamber. To this rule Valvata and Paludina alone form exceptions ; they 
have an excretory canal which opens by an orifice placed at the anterior 
edge of the mantle. The kidney is not a large gland, nor is it acinous. 
Its secreting epithelium is arranged on lamellae or trabeculae which form 
a spongy mass ; the lamellae are attached to the lateral walls and the 
urinary cavity is free in the centre. In freshwater Prosobranchs 
(Neritina^ Paludina') the glandular mass is, as a rule, considerably 
developed. It fills the urinary cavity, and the kidney has then the 
appearance of a large spongy body ; Cyclostoma and Valvata are 
exceptions to this rule. 
The glandular mass, that is to say the active part of the kidney, is 
divided into two glands which are quite distinct — the kidney properly so 
called, and the nephridial gland. The latter has almost always the 
form of a band which extends along the pericardium, and sometimes 
also along the base of the branchial cavity. Its colour is different from 
that of the kidney. It is hollowed out by a lacuna which is bounded, 
especially on the side nearer the urinary cavity, by a layer of powerful 
muscles, the function of which is to regulate the circulation in the 
organ. This lacuna is a diverticulum of the auricle, with which it 
freely communicates; it is partially obliterated by connective tissue, 
the essential elements of which are large cells ; these are arranged in 
the meshes of a plexus of stellate connective cells. The function of this 
gland is to modify the constitution of the blood either by pouring into 
it definite products or formed elements. It is, therefore, an organ of 
reserve or lymphatic gland. On the side near the renal cavity the 
nephridial gland is lined by an epithelium which projects into its 
interior, where there are branched canals which are always well 
separated from the blood-lacuna. The nephridial gland may be con- 
sidered as representing the left kidney of the Diotocardia. Patella is 
an intermediate form. 
In most of the Tasnioglossata the glandular mass of the true kidney 
is homogeneous in structure, but in a few (Natica, Cyprsea) there are 
signs of differentiation. The Stenoglossata have the organ divided 
into two lobes, and further modifications and differentiations may be 
traced. 
There are two types of renal gland-cells ; that of the first is to be 
seen in the Diotocardia. They are very generally ciliated ; sometimes 
they contain no foreign concretions, but at others the bodies excreted by 
the cell condense in the form of small concretions, of which there are 
