No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 417 



the same name. The peculiar apical sense organ in Crepidula, 

 I am satisfied, does not occur in Planorbis. In the former 

 genus the four cells which form this organ remain of small size, 

 acquire a tuft of long cilia, and become united later with a pair 

 of nerve cords which arise from the cerebral ganglia on either 

 side. The existence of such a structure in a molluscan larva 

 is a striking mark of relationship with the annelid trochophore. 

 The presence of this organ in the larva of Planorbis might 

 naturally be looked for, but the four apical cells which form 

 this organ in Crepidula become in Planorbis very much enlarged 

 and thinned out and form a part of the apical plate. The cells 

 which compose this plate are six in number, the four apical 

 cells just mentioned and a pair of cells lying in front of these. 

 Of the origin of this pair of cells I am not entirely certain, but 

 I think they are the cells k^'''^' and ic''"'''. These cells arose 

 from the divisions of the intermediate cells lying in the angles 

 between the arms of the cross. The cell lineage of the apical 

 plate in Planorbis may be expressed as follows : 



(apical cells \a''\ \b''-\ \c'-'\ id''-' 

 I intermediate cells ib'-'-'-\ ic'-'-"-' 



The anterior end of the apical plate is limited by the upper 

 median cell of the prototroch, and its posterior boundary is 

 formed by the basal cell of the posterior arm of the cross. 



The Cell Lineage of the Head Vesicle. 



Owing to the fact that the cells composing the head vesicle 

 in Planorbis are few in number and of large size, I have been 

 able to determine the exact lineage of all the components of 

 this structure. The head vesicle may be said to first appear 

 when the posterior trochoblasts and the cells of the posterior 

 arm of the cross begin to enlarge. This enlargement, which 

 begins before the lOO-cell stage, causes the upper pole to move 

 toward the anterior side of the ^gg. The tip cells of the lateral 

 arms of the cross enlarge rapidly, and, at a later period, the 

 cells irt'"^'', i^'"^^ lying just above the tip cells, also enlarge. All 

 of these cells, owing to the anterior rotation of the upper pole 



