692 EEPORT— 1891. 



9. The anterior septum does not form a complete partition between the two 

 sides of the cord. It is traversed by the transverse fibres of the commissures. 



10. The posterior septum is traversed by the transverse fibres of the posterior 

 commissure, but it ' forms a complete partition between the posterior white 

 columns. 



11. There is no posterior Jissure, and the posterior septum is not a septum of 

 pia-inater, but of spongioblastic fibrils ; it is, therefore, essentially a portio7t of the 

 cord substance, not of its sheath. 



12. The anterior fissure is formed in the usual manner, and contains a fold of 

 pia-mater. 



6. On the Innervation of the I]pipodial Processes of some Nudihranchiate 

 Mollusca. By Professor W. A. Heedsian, B.Sc, and J. A. Clubb. 



In 1889 one of us (W. A. Herdman) read a paper at the Newcastle-on-Tyne 

 meeting of the British Association on the structure and functions of the cerata in 

 Nudibranchs, in which these dorso-lateral processes were regarded as being pro- 

 bably epipodial outgrowths. In other papers published since we have compared 

 the conditions of these structures in various genera of Nudibranchs, and have 

 tried to show that they are all modifications of simple lateral epipodial ridges. 



The question has, however, been raised lately by Pelseneer and others as to 

 whether the so-called epipodia of mollusca are all homologous structures, and one 

 of the subjects of controversy now is the origin of the nerve supply in various 

 forms, it being supposed that where the processes are innervated from the pleural 

 ganglia they are pallial in their nature, and where supplied from the pedal ganglia 

 they are to be regarded as outgrowths from the foot. 



Cousequently, it seemed to us of importance to determine afresh the origin of 

 the nerves supplying the cerata in several diSerent types of Nudibranchiata, 

 especially as the results of former investigations, depending entirely, we believe, 

 upon minute dissection, are puzzling, and to some extent contradictory. We have 

 traced the nerves from the ganglia, by means of serial sections, in representatives 

 of the genera Folycera, Anctila, Tritonia, Dendronotus, and Eolis, with the follow- 

 ing results : — 



In Folycera quadrilineafa the cerebral and pleural ganglia are completely fused 

 to form a cerebro- pleural mass. The 'epipodial' nerves are found arising from 

 the ventral and posterior part of this mass {i.e. distinctly from the pleural ganglia), 

 and they run along the sides of the back to supply the ceratal ridges. 



In Ancula cristata the pleural ganglia are fairly distinct from the cerebral. In 

 a specimen cut into about 500 sections, we find in the 100th section or so from the 

 anterior end six distiact ganglia (the cerebral, pleural, and pedal pairs) surrounding 

 the oesophagus. A few sections further back the cerebrals disappear, and then 

 the epipodial nerves are found arising from the dorsal edge of the pleural ganglia. 

 The nerves soon turn posteriorly, and then give off their first branches dorsally. 

 These branches enter the mesoderm of the body wall, and can then be traced back 

 through over a hundred sections to the first pair of cerata, which they enter. The 

 main nerve passes back to the remaining cerata. 



In Tritonia and Dendronotus also the epipodial nerves arise from the pleural 

 ganglia ; but in Eolis (or Facelina) coronata we find that the main nerves to the 

 cerata arise distinctly from the pedal ganglia. We have also traced in the same 

 series of sections the ordinary pedal nerves to the foot proper, so there can be no 

 question as to the nature of the ganglia from which the nerves arise. The 

 epipodial nerves spring from about the middle of the pedal ganglion, rather on the 

 dorsal surface, and, after a short course, pass through the muscular layer of the 

 body wall and are distributed to the clumps of cerata. 



But in addition to these main epipodial nerves in Eolis, we find also a nerve 

 arising from the compound ganglionic mass, immediately ventral to the eye 

 (probably therefore from the pleural element), which goes to the front cerata. 

 This pleural nerve has its origin distinctly anterior to the origin of the main 

 epipodial nerves from the pedal ganglia. , 



