503 



containing muscle-fibers, they may pump water. They are not nearly 

 as well adapted for the purpose as the gills of Yoldia. The plates 

 are without dorsal projections and have a general ventral slope. The 

 absence of siphons is also important in this connection. As the animals 

 live buried in the mud and have comparatively open mantle-chambers 

 strong currents of water would not be especially useful. 



Excretory Organs. 



The excretory organs of adult animals are very extensive. Each 

 consists of a convoluted tube, the walls of which are much sacculated. 



In Yoldia the inner 

 (pericardial) and outer 

 (mantle-chamber) openings 

 of each excretory organ 

 are very near each other. ie 

 The inner end, Fig. 6 ie, 

 leaving the pericardium, 

 turns abruptly toward the mc. 



Fin. 6. Reconstruction of the ends of the left excretory organ and genital duct of 

 Yoldia limatula. Viewed nearly from the ventral side, gd genital duct, ie inner (peri- 

 cardial) end of the excretory organ, oe outer end of the excretory organ, mco common 

 opening of the genital duct and excretory organ into the mantie-chamber. 



middle line of the body, crosses the outer end, oe, and is continued 

 anteriorly close to the pericardium. After passing through many sac- 

 culated loops, that extend well into the foot, posterior to the stomach, 

 the tube again comes near the pericardial wall, enlarges, runs along- 

 side the anterior turn of the inner end, narrows down again, and 

 opens into the mantle-chamber through an antero-posteriorly elongated 

 opening, mco, just posterior to the point where the outer and inner 

 ends of the tube cross. 



The genital duct, gd, following the course of the cerebro-visceral 

 commissure, turns abruptly when almost in contact with the inner end 

 of the excretory organ, meets the outer end, and opens with it. 



In both species of Nucula, much the same arrangement exists. 

 The genital ducts open with the outer ends of the excretory organs, 

 and not near the pericardial openings, as Pelseneer (7) has de- 

 scribed for some members of this group. No indication of the gono- 

 pericardial canal, described by Stempell (10) for Leda sulcata, has 

 been observed. 



