i8 



NERVOUS SYSTEM AND SENSE-ORGANS 



Fig. 1022. Central Nervous Systems of Pond- 

 Snail (Limnteus, A) and Garden-Snail (Helix, B), 

 diagrammatic and enlarged 



Br.G., Brain ganglia; Ped.G., pedal ganglia; 

 S.G., side ganglia (dotted). Above are seen the 

 small buccal ganglia connected by pharyngeal 

 nerves {Ph.} with the brain ganglia, while below, 

 the short nerve-loop with its ganglia is represented 

 (darkly shaded). 



two side-ganglia are connected together, and in the course of 

 which are three ganglia supplying many of the organs of the 



body. As in all Gastropods, the 

 upper part of the body of the River- 

 Snail has been subjected to a sort 

 of twisting process, as the spiral 

 shell suggests, and this has affected 

 the nerve-loop, making it 8-shaped, 

 as shown in the figure. This well- 

 specialized central nervous system is 

 associated with the presence of a 

 clearly defined head, while just the 

 contrary is the case in a Mail-Shell. 

 Centralization has taken place to a 

 still greater extent in some of the 

 Gastropods, e.g. in the Pond- Snail 

 (Limn&us) and Garden- Snail (Helix 



aspersa, fig. 1022), where the nerve -loop, which here has not 

 been inflenced by the twisting of the body, is so short that its 



three ganglia are closely approxi- 

 mated to one another, and also to 

 the foot-ganglia and side-ganglia 

 of the nerve-ring. 



Both in Bivalve Molluscs (La- 

 mellibranchia) and Tusk - Shells 

 (Scaphopoda) the nervous system 

 follows the type described for 

 Gastropods, but is less concen- 

 trated, and the brain-ganglia are 

 relatively small, which may be 

 correlated with the absence of 

 any definite head in the former 

 group, and its imperfect develop- 

 ment in the latter. 



Among the Head- Footed Mol- 

 luscs (Cephalopoda) various de- 

 grees of concentration are found, 

 there being the least amount of 



this in the Pearly Nautilus, which is a primitive and isolated 

 type, while in the active Cuttle- Fishes, Squids, and Octopi cen- 



Nerve-hop 3 



Brain 



SideG. 



Foot G: 



Ph.G.- 



Fig. 1023. Central Nervous System of Cuttle- Fish 

 (Sepia], seen from the right side 



G., Ganglia; N. t nerve; Pk.G., pharyngeal or 

 buccal ganglia; Ospk.N., osphradial nerve of water- 

 testing organ (Osphradium}. 



