.202 BULLETIN OF THE 



With this I cannot agree, although I admit that at the time when 

 the groups of cells which form the ganglia begin to be proliferated from 

 the ectoderm, it is extremely difficult to distinguish them from the 

 mesodermic elements (Plate I. Fig. 5). It is to be observed, however, 

 that Fol considers it an unimportant distinction, whether the ganglia 

 are formed from groups of mesodermic cells which have themselves 

 recently originated from the ectoderm, or by a proliferation of cells 

 directly from the ectoderm. 



I am unable to reconcile the account of the development of the pedal 

 ganglia in Bithynia given by P. Sarasin ('82, pp. 47-49), with the 

 conditions seen in Limax ; nor can I think it probable that any consid- 

 erable difference exists between nearly related mollusks in regard to 

 the place whence the ganglionic cells arise. Sarasin maintains that in 

 Bithynia the pedal ganglia arise from a single median thickening of the 

 ectoderm of the dorsal wall of the foot, in the region where that wall 

 bends over to become continuous with the posterior wall of the visceral 

 sac. Anteriorly, in the region of the oral invagination, this median 

 band of cells forks, and each branch becomes joined to the correspond- 

 ing cerebro-pleural cell mass by a slender cord of cells. Subsequently, 

 the posterior unique portion of the proliferated cell mass is completely 

 divided into lateral branches by a separation which progresses from in 

 front backward. It seems to me that, according to this account, both 

 the pedal ganglia must be regarded as arising from a common mass of 

 cells, and that they are not from the beginning wholly separate, as I 

 maintain for Limax. 



The relative positions of pedal ganglia and otocysts present, to my 

 mind, a serious objection to Sarasin' s view, which may not have seemed 

 so important to him on account of his uncertainty about the origin of 

 the otocysts. I believe it is sufficiently evident that the otocysts do 

 not arise, as Sarasin thinks probable, from the cerebro-pleural prolifera- 

 tions, but independently, and from the dorso-lateral wall of the foot in 

 the region of the " pleural groove." They ultimately lie immediately 

 dorsal to the corresponding pedal ganglia. If Sarasin's view as to the 

 origin of the pedal ganglia as a median dorsal proliferation were correct, 

 the ganglia would have to migrate to a lower plane than that occupied 

 by the otocysts. But there is no evidence either in Limax or the figures 

 given of Bithynia which would confirm such a supposition. As further 

 corroboration of my opinion that the pedal ganglia arise from the ventral 

 and lateral walls of the foot, I would cite the conclusion reached by 

 Salensky ('86, pp. 691, 692) for Vermetus. He has shown that the 



