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 aerated 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 lamellae 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. 



