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 ganglia develop. 
The tip cells of the lateral arms certainly do not take part in the forma- 
tion of the gangtia, as they lie 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 ganglionic rudiments. Comparing, however, the above 
approximate derivation with other Mollusks 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 ganglia in Planorbis, as here they are surrounded 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 stomodzal invagination, 
and at a stage shown 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 
which connected the otocysts with the ectoderm, and in part from other 
cells proliferated from the ectoderm in the same region. At first the 
cerebral ganglia 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), 
