MUSEUM OF COMPARATIVE ZOOLOGY. 201 



The Sarasins ('88, p. 57) consider these clusters of cells homologous 

 with the " taste-buds " and " lateral organs " of vertebrates, and say that 

 they are to be found in and at the margin of the Sinnesplatten, and 

 along the sole of the foot, — more rarely on the sides of the foot. I 

 think these organs are probably the same as those which I have seen in 

 Limax, and to which I attribute simply the function of contributing to 

 the formation of the ganglia. 



Salensky ('86, pp. 685-G90) describes the cerebral ganglia of Verme- 

 tus as arising from a pair of ectodermic thickenings, which early show 

 pocket-like invaginations, and become deeper and narrower. From tlie 

 inner ends of these invaginations are formed the main portion of the 

 ganglia. The latter are united to each other by a very small commis- 

 sure, composed of fibrous prolongations of the ganglionic cells surrounded 

 by other nerve cells. 



The principal difference between the method of development in Ver- 

 inetus and that in Limax maximus consists in the fact that the detach- 

 ment of the deep portion of the invaginations to form the ganglia in 

 Vermetus is not etfected until the invaginations have reached their ulti- 

 mate size, whereas in Limax the detachment of cells from the invagi- 

 nated area begins as early as does the invagination, and accompanies it 

 during the whole of its formation. 



Kowalesky ('83% pp. 1-54) found in Dentalium two deep invagina- 

 tions, which he calls the " sincipital tubes," one on each side of the 

 head region, a little ventral to the middle of the velar area. From the 

 posterior deep ends of these sacs the cerebral ganglia are subsequently 

 formed ; but he is uncertain whether all the cells concerned in the in- 

 volution share in the formation of the ganglia. If his Figure 65 is 

 compared with Figures 27 and 33 A in Salensky's paper, the close resem- 

 blance in the method of origin of the cerebral ganglia in the two types 

 becomes apparent. 



Fol ('80, pp. 169, 170) asserts that the pedal ganglia of the aquatic 

 pulmonates appear as condensations in an already formed mesoderm, 

 and that they are nearer the pharynx than the ectoderm when they 

 begin to be discernible. " One may therefore sa}"^," he adds, " that these 

 ganglia arise from the mesoderm without prejudging the unsettled ques- 

 tion, viz. from which of the primordial layers arises the mesoderm 

 which forms them." Of the pedal ganglia of the terrestrial pulmonates, 

 he says that they are diffentiated en lieu et place in the midst of the 

 mesodermic tissues of the foot. 



