MUSEUM OF COMPARATIVE ZOOLOGY. 199 



maxim us, where the abdominal ganglion is intimately fused with the 

 right visceral, and is also in close connection with the left visceral 

 ganglion. 



As was to have been anticipated, the abdominal ganglion of Limax 

 corresponds more uearly in position to that in Lymneus and other 

 fresh-water pulmonates, as described for the adult by Lacaze-Duthiers 

 ('72, pp. 437-500). 



Of the authors who have studied the origin and development of the 

 cerebral ganglia in Mollusks, Fol ('80, pp. 168, 169, 193-195) is the 

 only one who has pursued his investigations on Limax maximus. 

 He says ('80, p. 193): "Vers l'epoque de la fermeture de la vesicule 

 oculaire, se montrent deux autres enfoncements de l'ectoderme. L'un 

 des deux, assez vaste et situe a la base du tentacule, a son bord interieur, 

 est l'origine du ganglion cerebro'ide ; je le decriai plus loin. L'autre 

 enfoncement, plus petit, est situe au-dessous de ce dernier, a la base du 

 pied, et mene a la constitution de la vesicle auditive." 



As to the method by which the cerebral ganglia originate, this agrees 

 in part with that which I have found ; but as to the time of origin, my 

 investigations lead me to a different conclusion. The otocysts are pres- 

 ent as small groups of cells (Plate I. Fig. 4), and the cellular elements 

 which go to form the beginning of the pedal ganglia are also being pro- 

 liferated (Fig. 3), before there is a trace of the invaginations which go 

 to form the cerebral ganglia (Fig. 2). 



A little later the otocysts assume the form of closed vesicles, uncon- 

 nected with the ectoderm (Plate I. Fig. 9), while the cerebral invagi- 

 nations are now seen as shallow pits (Fig. 6). Therefore, in Limax 

 maximus the formation of both pedal ganglia and otocysts precedes 

 that of the cerebral invaginations. 



Sarasin ('82, pp. 1-68) maintains that in Bythinia tentaculata there 

 are no invaginations to form the cerebral ganglia. They arise as thick- 

 enings of the ectoderm, one on each side of the body, which he calls 

 die Sinnesplatte. 



In the recent researches of the Sarasin brothers ( ? 87, pp. 600-602, 

 '88, pp. 59-69) on Helix Waltoni, of Ceylon, it is asserted that each of 

 the cerebral ganglia is at first represented by a group of cells derived 

 from the part of the ectoderm called " Sinnesplatte " before there is 

 any invagination. There are two groups of these cells, one on each side 

 of the body. Somewhat later two infoldings arise from each Sinnes- 

 platte, one above the other. These infoldings become long, narrow 



