=e 
238 HELIX ASPERSA. 
INTERNALLY, the NERVOUS SYSTEM! in this and other highly-organized species 
is better and more conveniently studied in immature animals when the component 
ganglia of the nerve ring are most easily distinguishable, as at maturity the separate 
parts become fused together and their limits indistinct. 
sd 
3T — a= Ta ph 
4 L ha ; Fic. 308.— Nerve centres from a half-grown Helix 
a asfersa, to illustrate the relative positions of the-various 
é \ ganglia and the origin and distribution of the chief nerves; 
highly magnified. 
we. fe-be 
a. abdominal ganglion; »2. and 24. right and left 
buccal ganglia; 7c. and Zc. right and left cerebral 
ganglia ; 7». and 2.4. right and left pedal ganglia; ».A/. 
and /.f/. right and left pleural ganglia; »w. and Zz. 
right and left visceral or pallial ganglia ; c-6.c. cerebro- 
buccal connective ; c-f.c. cerebro-pedal connective ; c-f/.c. 
cerebro-pleural connective; c¢.c. cerebral commissure ; 
a.p. anterior pedal nerve; g. nerve to ovotestis, with /. 
branch to heart; 2. nerve to lips; #2. to anterior and 72’ 
to posterior portion of mantle; 0. to ommatophore; @. to 
cesophagus and stomach; #. to penis or male organ ; pA. 
to pharynx ; #.f. to posterior part of foot ; s.d. to salivary 
m : 
ducts ; ¢. to lower tentacle and lips. 
aed | 
G h } pp 
hog 
The CEREBRAL GANGLIA or brain is much blended in adults, and bilobed or 
cleft at the outer margins, the parts are united by a very stout COMMISSURE, and 
give nerves to the sensory organs. The PEDAL GANGLIA are a pale of closely- 
apposed nervous masses. this approximation being so intimate that the commissure 
is to all appearance quite lost. The VISCERAL or PARIETO-SPLANCHNIC centre is 
constituted by five medullary masses, which, though more distinct when young, 
become in adult life agglomerated together, not only amongst themselves, but 
with the neighbouring pedal centre; this visceral centre not only innervates the 
body wall, the mantle, and the viscera, but controls the sympathetic nerves which 
regulate the involuntary motions of the various organs. The BUCCAL or STOMATO- 
GASTRIC GANGLIA are small and widely-separate nerve masses, which do not fuse 
together ; they innervate the mouth, stomach, ete., and are joined to the brain by 
pigmented connectives. 
The OLFACTORY, the VISUAL, and the AUDITORY ORGANS are each well developed. 
The MUSCULAR SYSTEM is most strikingly exemplified in the COLUMELLAR 
RETRACTOR, which is the most important muscle of ; 4 
the body, and is affixed at its distal end to the f 
columella of the shell, near the commencement of 
the penultimate whorl, passing beneath the Inng 
an 
Tf. y : 
chamber, and dividing into a multitude of fibres f 
which interlace with the tissues of the foot ; near f 
its origin, this muscle gives rise to the paired Ne 
TENTACULAR RETRACTORS, each of which about \ 
midway of its course gives off a broad and x 
powerful tripartite muscle to the anterior part of 
the foot; the retractor of each ommatophore ex- 
pands considerably within the tentacles, and gives 
off a branch to the lower tentacle of its side, which 
sends a slip to the lips; the PHARYNGEAL RETRACTOR 
is a powerful ribbon-like muscle, which also origin- 
ates from the columellar muscle adjoining to and 
Fic. 309.—Cephalic retractors of 
Helix aspersa, X 3. 
c.m. columellar muscle ; % anterior 
foot retractors; 4. ph uryngeal or buc- 
immediately in advance of the paire:l tentacular 
muscles, and divides before reaching the buccal 
bulb, to which it is attached laterally and ventrally by divided and expanded ends. 
The LYMPHATIC GLANDS are not strictly localizel in H. aspersa, but diffused 
amongst the connective tissue in various parts of the body, but especially about the 
pulmonary vessels. 
The PEDAL GLAND is, according to Prof. André, similar to that of H. pomatia, 
but has an unique peculiarity amongst our indigenous pulmonates in having the 
three median folds on the roof of the excretory canal, densely covered with very 
short cilia, about 0-004 mill. in length. 
1 Monog. i., p. 147, et seq. 
cal retractor; ¢, tentacular retractors. 
