1904.] NATURAL SCIENCES OF PHILADELPHIA. 373 



Later many of these large cells also divide and go into the ganglia. 

 Thus it will be seen that the two cerebral ganglia arise from elements of 

 the two lateral arms, the anterior rosettes, and probably also from some 

 cells of the anterior arm which have been pushed laterally by the 

 advance of the apex and lie in the region where the gangUa develop. 

 The tip cells of the lateral arms certainly do not take part in the forma- 

 tion of the ganglia, as they he too far laterally and probably go into 

 the velum. Where no large cells, the definite lineage of which is known, 

 are left as landmarks, it is obviously impossible to give absolute deri- 

 vatives for the ganghonic rudiments. Comparing, however, the above 

 approximate derivation with other MoUusks which have been studied 

 in this connection similarities are evident. In Crepidula the ganglia 

 "very probably arise from the lateral extensions of the anterior arms". 

 Holmes has been able to state very definitely the manner of origin of 

 these gangHa in Planorbis, as here they are smTOunded by conspicuous 

 cells. He says: ''The tip cells of the lateral arms and the cells lying 

 immediately above them do not enter into the formation of these 

 masses; with the exception of these, two cells in each arm, all the cells 

 in the lateral arms of the cross, the cells of the anterior arm, except the 

 tip and basal cell, and the central region of the cross, except the four 

 apicals, and the two cells lying in front of them, enter into the forma- 

 tion of these rudiments". 



Otocysts and Pedal Ganglia. — The otocysts appear at a consider- 

 ably earlier period than the ganglia which innervate them or the 

 cerebral ganglia. They are first seen as slight invaginations on 

 the sides of the foot slightly below the stomodaeal invagination, 

 and at a stage sho^^-n in figs. 103 and 104 have developed to 

 deep pits, the openings of which have become much constricted. As 

 these constrictions narrow, the two otic vesicles arise and are con- 

 nected with the external ectoderm by strands of cells which re- 

 sulted from the constriction of the outer portion of the invaginations. 

 Somewhat later the pedal ganglia are seen slightly external to the 

 otocysts in position. These ganglia arise in part from the strands 

 wliich connected the otocysts with the ectoderm, and in part from other 

 cells proliferated from the ectoderm in the same region. At first the 

 cerebral gangha are not connected with each other by a commissure 

 nor with the pedal ganglia, but later cells grow out and meeting con- 

 nect the cerebral ganglia together, while between cerebral and pedal 

 ganglia like connectives arise, probably both ganglia contributing 

 cells to their formation. These connectives are very large (fig. 94), 



