242 OILMAN A. DREW. 



is thus able to leave little of its surface exposed to the attacks of 

 enemies. The other has developed one of the most complicated 

 of machines in connection with its mouth to aid it in getting food. 

 As the head apparatus of the one type has increased in com- 

 plexity, there has been greater need of ganglia to supply it, but 

 in the whole line of development of the other type there has 

 been no complicated head apparatus. About all of the actual 

 evidence that we have of the presence of pleural ganglia in lamel- 

 libranchs is that given by Pelseneer, 1 who finds in Nucnla and 

 some other forms, that each anterior ganglionic mass is so shaped 

 that it is possible to consider it as two ganglionic masses, and 

 further that the connective that runs from this mass to the pedal 

 ganglion is connected with this mass by two roots. The inter- 

 pretation that he has put on this is that the two apparent divisions 

 of the ganglion represent respectively the cerebral and pleural 

 ganglion, and that the roots of the connective represent the 

 cerebro-pedal and pleuro-pedal connectives that have become 

 fused before reaching the pedal ganglion. My own view, dis- 

 cussed in another paper 2 is that the apparent division into two 

 ganglionic masses is superficial, and due to the swellings accom- 

 panying the origins of nerves, and that one of the cerebral ends 

 .of the connective may be the central end of the otocystic nerve 

 which is fused for the greater part of its length with the connec- 

 tive, but, unlike most forms, is free near the ganglion. This view- 

 seemed to me most reasonable as Stempell 3 has found that in 

 Solcynia togata, a supposed near relative of Nitcula, the otocystic 

 nerve arises directly from the cerebral ganglion and is separate 

 from the connective throughout its length. So far as I know, 

 the instance given by Stempell is the only one that has hereto- 

 fore been reported where the otocystic nerves originate from the 

 cerebral ganglia, and are free from the cerebro-pedal connectives 

 throughout their length. Pecten temiicostatus has the same 

 arrangement. In this form the position of the ganglia, connec- 

 tives and otocysts is such that it is a very simple matter for the 



JPelseneer, "Contribution a 1'etude des Laruellibranchs," Arch, de BioL, XL, 

 1891. 



2 Drew, "The Life-History of Nucula delphinodonta" Quart. Jour, of Micro. 

 Sfi., Vol. 44, Part 3, New Series, 1901. 



'Stempell, " 7-ur Anatomic von Solemya togata" Zool. Jahrb., Bd. XIII. , 1899. 



