PHYLUM MOLLUSCOIDA 



345 



object. The aboral side of the larva becomes greatly extended, so 

 that almost the entire integument of the primary zooid is devel- 

 oped from this part (i.e. from the region occupied by the retractile 

 disc and pallia! groove). Accompanying the extension of the 

 aboral surface are the obliteration of the pallial groove and the 

 bending down of the corona towards the oral side. Thus the stage 

 of the larva termed the umbrella- shaped stage is reached. The 

 sucker is everted, and by means of it the larva becomes attached. 

 The edge of the " umbrella " becomes bent downwards, and 



end. 



end 



cent 



ect 



eel 



FIG. 277. Early stages in the development of Bugnla. cent, central mass of cells ; cor. corona 

 ect. ectoderm ; end. endoderm ; seg. segmentation-cavity. (After Vigelius.) 



fused wijh the broad plate into which the sucker has ex- 

 panded, thus enclosing a circular cavity, the so-called vestibule 

 (Fig. 279, v). The walls of this, consisting of the coronal cells 

 and a portion of the original sucker, become broken up and the 

 cavity is merged in the general cavity in the interior of the 

 larva. All the larval structures have now disappeared with the 

 exception of the basal plate of the sucker and the retractile disc. 

 The former gives rise to the basal part of the wall of the primary 

 zooecium. From the latter, which becomes invaginated, or from 

 a sac which is developed to replace it, are developed both 

 the ectodermal and endodermal structures of the primary zooid. 



