414 MARINE INVERTEBRATES 
animal, for there are two sets, one upon each side of the body, 
or visceral mass, and lying between the body proper and the 
mantle folds. Each set consists of two plate-like bodies with a 
texture of reticulated or basketwork appearance. Thus there 
are the inner and outer right gills and the inner and outer left 
gills. If one gill is removed and carefully examined it will itself 
probably be found to be double, consisting of many filaments 
placed side by side and then doubled back like a row of hair- 
pins, the filaments being united by interciliary processes, or by 
vascular channels together with more or less dense connective 
tissue. 
The modifications of the pelecypod gills are difficult to follow, 
but the principle upon which they perform their duties is the 
same in all cases. The gill-filaments are all connected with a 
long vein, and, being hollow, admit the blood, which is aérated by 
close contact with the water and is then returned to another vein 
in immediate connection with the auricles of the heart. There is 
another function accomplished by the gills, which in some fami- 
lies seems to be quite as important as their respiratory one, 
namely, the office of giving lodgment to the ova while in process 
of development before hatching. At certain seasons the gills of 
a number of pelecypod genera become literally filled with eggs; 
sometimes this curious phenomenon extends to the mantle itself, 
and more or less to the entire surface of the animal. The eggs 
are first regularly ejected from the genital ducts and find lodg- 
ment upon the body-surface, but usually only upon the gills, 
where they remain as in a brood-pouch between the lamelle of 
the inner and outer gills) When the eggs hatch, the free-swim- 
ming young escape from the mantle cavity. Probably not more 
than one individual in a million ever reaches maturity. 
The figure on page 408 shows the mantle removed, exhibiting 
the gills, the foot, the labial palps, and the pericardium, inside of 
which is the heart. Removing the gills, we find exposed the body, 
or visceral mass, which, as in the Gasteropoda, is thickened below 
into the foot. A longitudinal section is shown on page 408, the 
visceral mass being sliced almost through the middle; a portion 
of the gills of the farther side shows below. 
