18 NERVOUS SYSTEM AND SENSE-ORGANS 
two side-ganglia are connected together, and in the course of 
which are three ganglia sup 
Fig. 1022.—Central Nervous Systems of Pond- 
Snail (Lzsnceus, a) and Garden-Snail (Hedéx, 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 (PA.) with the brain ganglia, while below, 
the short nerve-loop with its ganglia is represented 
(darkly shaded). 
plying 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 
(Limneus) 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 
Oop. GN, 
“Mantle G. 
“--""Mantle.N. 
Nerve-loop------- 
Fig. 1023.— Central Nervous System of Cuttle-Fish 
(Sepia), seen from the right side 
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- 
G., Ganglia; WV., nerve; P%.G., pharyngeal or luscs (Cephalopoda) various de- 
buccal ganglia; Osfh.N., osphradial nerve of water- 
testing organ (Osphradium). 
this in the Pearly Nautilus, 
type, while in the active Cut 
grees of concentration are found, 
there being the least amount of 
which is a primitive and isolated 
tle-Fishes, Squids, and Octopi cen- 
