MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 409 



however, not taking exactly the same path. It will be 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 

 ill Area iJexata (Fig. 06, PI. Lxxxviii). 



The blood corpuscles of Area are colored by haemoglobin, and I have attempted 

 to trace the circulation in the fllainents. I removed one valve of the shell, placed the 

 animal in water, and examined the gill by separating, without 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 lamelLT the currents wcmld 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 lamellibrauchs, 

 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 GROAN. 



The uephiidium is situated immediately beneath the pericardial chamber. The 

 more primitive condition of the gland is preserved in Nucula and iSolenoniya, 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 eiiithelium 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 Irom 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 looj) 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. 10, PL 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, Rankin (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 



