MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 
409 
however, not taking exactly the same path. It will he noticed that in gills of this 
kind the blood space of the filament is divided by a septum or is greatly flattened 
out and shows in section a long, narrow blood channel. This latter condition is found 
in Arcapexata (Fig. 66, PI. lxxxviii). 
The blood corpuscles of Area are colored by haemoglobin, and I have attempted 
to trace the circulation in the filaments. I removed one valve of the shell, placed the 
animal in water, and examined the gill by separating, withoxit injury, a few filaments 
at a time, as they are connected with one another only by the ciliated junctions on 
their sides. Under the microscope the pale yellow corpuscles could be seen in motion 
in the blood stream. In both outer and inner lamellae the currents would keep up a 
constant flow outward for half a minute, at times, though the rate of the current did 
not remain constant. The streams would become slower, finally stop, and then a back- 
flow would set in. After a short time these ceased, and the currents resumed their 
original course. I did not confound two separate currents, but could see individual 
corpuscles being carried in one direction and then back in the other. 
The currents in the different filaments were independent of one another, and 
there seemed to be a somewhat irregular channel for them. 
In cases where the filaments are joined to the mantle and the ventral side of the 
visceral mass ( Ostrea ) the blood may be finally collected in vessels running along the 
line of this concrescence. The currents through such filaments can not be at all defi 
nite, as the filaments open into one another at various places. 
It seems probable that the gill is used as a respiratory organ in all lamellibranchs, 
though in a few very little blood, apparently, gets into them. In some cases the other 
surfaces of the body, and particularly those of the mantle, may play a more important 
part than the gills in the aeration of blood. 
THE EXCRETORY ORGAN. 
The nephridium is situated immediately beneath the pericardial chamber. The 
more primitive condition of the gland is preserved in Nucula and Solenomya , where it 
is a simple tube, bent upon itself. One end of this opens into the pericardium, the 
other to the exterior by the branchial chamber. The lining epithelium in these cases 
is similar in all parts of the tube and is all secretory (Pelseneer, No. 17). It consists 
of large cuboidal, vacuolated cells, without concretions. The generative gland opens 
into this kidney and near its pericardial opening. There are two excretory organs, 
one on each side, having no connection with one another. 
There are many variations from this simple loop, but the general plan is always 
followed. In most cases the loop becomes differentiated, so that its terminal half 
becomes nonglandular, while only the half connected with the pericardium remains 
glandular. On account of the loop in the organ the glandular portion is ventral and 
the nonglandular is dorsal in vertical sections. The gland is seen in section beneath 
the pericardium in Fig. 16, PI. lxxxii. gl represents the large glandular portion 
and ( ngl ) the nonglandular tube above it. 
The glandular portion possesses greatly folded walls, bounding its lumen, for the 
increase of the secreting surface. In Anodon, Eankiu (No. 20) has described the 
lining cells as being more or less ciliated. 
In Pecten, the nephridia are situated beneath the adductor muscle (Fig. 46, PI. 
lxxxvi, n) and are exposed to the exterior, as they hang in the mantle chamber. They 
