THE LIFE-HISTOHY OF NtJCULA DELPHINODONTA. 357 



Nucula are not as effective as those of Yoldia, but the method 

 oi: life makes observation more difficult. As suggested by 

 Mitsukuri (11), it seems probable that the large palps with 

 their numerous large blood-spaces may be important respira- 

 tory organs. 



Gills. 



A short time after the embryo sheds its test, a portion of 

 each lobe of the mantle near its posterior border begins to 

 thicken (fig. 39, g.) and then to project anteriorly. These 

 thickenings are the beginnings of the gills. They grow 

 rapidly, acquire cilia, broaden dorso-ventrally, and each 

 begins to divide into two lobes (fig. 40, g.). The formation of 

 the lobes is due to unequal growth more than to constriction. 

 Each lobe is at first a little knob that is flattened slightly 

 laterally. As growth proceeds the ventral lobe broadens 

 and flattens along its anterior border preparatory to the 

 formation of another lobe. Coincident with these changes 

 in the ventral lobe, the dorsal lobe grows anteriorly, and 

 forms a rather long finger-like process or filament, that 

 closely resembles the filaments of the developing gills of other 

 Laraellibranchs (fig. 41, g.). New lobes are added to the 

 gill by the unequal growth and division of each ventral lobe 

 in its turn, and as the new lobes are formed the more dorsal 

 lobes lengthen. 



Throughout life the gill occupies a decidedly dorso-ventral 

 position, but growth carries the ventral end some distance 

 toward the posterior end of the animal, so that the adult gill 

 lies somewhat diagonally (fig. 48,^.). In Yoldia (Text-fig. L) 

 the gills lie more nearly parallel to the long axis of the body. 



The chitiuous support of the gill makes its appearance 

 when the gill is still in the two-lobed condition. At first it 

 consists of a thin plate lying just beneath the epithelium on 

 the anterior border of the gill, and is continued from one 

 lobe into the other. Its ends lie near the anterior extremity 

 of each lobe. As the ventral lobe flattens the chitiuous 

 plate is extended along its anterior border, so that with the 



