THE LIFE-HISTORY OF NUCULA DELPHINODONTA. 329 



plate is thrown away, but this may not be the case. The 

 test cells may or may not remain attached to the apical plate 

 and stomodaeum until these are thrown away. Generally 

 many of them break loose or go to pieces before this change 

 occurs, but some of them nearly always remain. 



After casting is completed (fig. 35) the stomach (do.) and 

 the liver lobes (I.) are drawn further into the schizoccele, and 

 the liver lobes begin to be flattened out against the mantle. 

 The cerebral ganglia (eg.) lie almost directly posterior to 

 the anterior adductor muscle (aa.), and the ectodermal 

 thickenings that result in the formation of the pedal and 

 visceral ganglia soon begin to form (fig. 36). 



Thus far in the development of the animal the shell-valves 

 have remained gaping, but after the removal of the apical 

 plate and the stomodaeum they are free to close. This is 

 effected by the contraction of the anterior adductor muscle, and 

 materially diminishes the space between the shell-valves. 



The closing of the shell is accompanied by important 

 changes in the liver pouches, changes similar to those that 

 have been described for Yoldia (1). Apparently as the result 

 of the mechanical pressure the liver pouches go to pieces, 

 and the large cells of which they were composed become 

 rounded and scattered through most of the schizocoele 

 (fig. 36, z,). The posterior portion of the schizocoele is not 

 filled by the scattered liver-cells. This persists and finally 

 becomes the pericardium. 



The foot (fig. 39, /.) grows and soon executes feeble move- 

 ments. The pedal ganglia (pg.) and visceral ganglia {vg.) 

 take on definite form ; the posterior adductor muscle (pa.) 

 appears ; and the invaginations that result in the formation 

 of the otocysts are formed. Yery possibly commissures 

 connect the ganglia at this time, but I have not been able to 

 distinguish them from the surrounding tissue until a some- 

 what later stage. A thickening on the inner surface of the 

 posterior end of each lobe of the mantle indicates the begin- 

 ning of the formation of the gill (fig. 39, g.). 



About this time a little invagination on the mid -line of the 



YOL. 44, PART 3,- — NEW SERIES, Y 



