THE LIFE-HTSTOEY OF NUCULA DELPHINODONTA. 875 



Otocysts. 



The otocysts are formed soon after the embryo sheds its 

 test. They originate as invaginations in the body-wall, a 

 little posterior and dorsal to the pedal ganglia. The invagi- 

 nations deepen and close over to form what seem to be closed 

 sacs, that soon come to lie near the pedal ganglia in the inte- 

 rior of the foot. As in the case of Yoldia, these sacs are 

 apparently entirely closed. Soon after the otocysts are 

 formed, before the gills acquire their second lobes, otoliths 

 appear. The otoliths have the appearance of little crystalline 

 fragments, but I am inclined to think that they are formed 

 in the otocysts, and are not introduced through the otocystic 

 canals, as has been held by some writers. The particles seem 

 to be too large to have been introduced through canals that, 

 at this stage, I am unable to find. Again, the otocysts never 

 seem to contain diatoms. Diatoms are very abundant in the 

 brood-sacs in which the embryos are carried, and form a 

 large part of the animal's food. Many of them are well 

 shaped to pass through small openings, and one would 

 expect to find them occasionally in the otocysts, if the con- 

 tained material consists of foreign bodies that have gained 

 access through the otocystic canals. 



About the time that the gills acquire their sixth pair of 

 plates the otocysts can be seen to be connected with the sur- 

 face of the foot (figs. 46 and 64, ot.). At first the connection 

 seems to be solid, but a little later openings can be traced 

 from the otocysts to the exterior. These tubes, the otocystic 

 canals, are quite slender near the otocysts, but widen toward 

 the surface of the foot. From each otocyst the canal passes 

 anteriorly, laterally, and a little dorsally to open to the 

 exterior (figs. 46 and 64, ot.). 



The position of the external opening is not just what 

 might be expected if the otocystic canals are remnants of the 

 invaginations that formed the otocysts. The otocysts are 

 formed just posterior and a little dorsal to the pedal ganglia. 

 As they develop, they sink into the interior of the foot and 



