no CONKLIN. [Vol. XIII. 



bodies were extruded. The apical cells, like many of the sur- 

 rounding cells, are covered by a coat of fine cilia, but there is 

 no bunch of very large cilia at this point, as in many of the 

 trochophore larvae. An apical sense organ has not hitherto 

 been found in molluscan larvae, I believe. 



(c) The Cerebral Ga?iglia and Eyes. — These ganglia are 

 formed on each side of the upper hemisphere, just apical to the 

 row of velar cells and about midway between the anterior and 

 transverse arm, Figs. 78 and 79. Before they begin to form, the 

 cells in this region become quite small by repeated divisions. 

 The method in which the ganglia are formed is shown in sec- 

 tion in Figs. 94 and 96, where it is seen that the cells prolifer- 

 ate inward from the surface, and thus form a solid aggregate 

 of cells. Over the area where the ganglia are being formed 

 the ectoderm is slightly depressed, but there is no invagination. 



From the position of these ganglia on each side of the ante- 

 rior cell plate, and in front of the cells derived from the trans- 

 verse arms of the cross (Fig. 79), it is very probable that they 

 arise from the two anterior rosette series and perhaps in part 

 from the lateral extensions of the anterior arm. In the larva 

 they lie on the ventral side of the coronal plane, and it is there- 

 fore probable that they are formed from cells lying originally 

 on the anterior side of the apex. 



There are scarcely any data for determining the cell origin 

 of these ganglia in other animals. Von Wistinghausen ('91) 

 states that they are the only derivatives of the first quartette 

 of ectomeres, but Wilson ('92) has shown that this is altogether 

 improbable. In Nereis, Wilson derives these ganglia from a 

 broad cell plate (see his Fig. S6 and Diagram 5) running across 

 the apical pole in a coronal direction and extending as far down 

 on each side as the prototroch. The position of this plate is 

 strikingly like that of the ganglia, commissures, and apical 

 organ in Crepidula as shown in apical view. Fig. 79 ; there is 

 scarcely a doubt that these three organs in Crepidula are homo- 

 logous with the "cephalic neural plate" in Nereis. 



The eyes are formed in connection with the cerebral ganglia 

 as independent involutions of the ectoblast. They lie, as 

 shown in Fig. 104, on the outer side of the cerebral ganglia 



