NERVOUS SYSTEMS OF INVERTEBRATES 17 
placed at a higher level, and uniting with one another behind 
above the intestine. The nerve-cells are distributed pretty uni- 
formly throughout both ring and cords, 
in the course of which are no distinct 
ganglia. The pharynx with its rasping 
organ receives branches from the nerve- 
ring, which do swell into small ganglia, 
and this is also the case with a pair of 
nerves running from the lateral cords to 
the under side of the stomach (see figure). 
In this sluggish animal digestion is the 
dominant function, and that is possibly 
why the only distinct ganglia in the ner- 
im) 
A/G 
/ 
i 
i 
i 
MY 
y 
We 
«wid 
( 
L 
Fig. 1020.—A Mail-Shell (Chiton) dis- 
vous system are related to the digestive _ sected from above, to show Central Ner- 
organs. The visceral nervous system con- 
vous System 
M., Mouth; /.A., intestinal aperture; 
sists in this case of (1) the nerves which 2-4. nerverings P.C., pedal cord; 2.C., 
lateral cord; Sz., stomach nerve passing 
run from the nerve-ring to the pharynx, — back to pair of gastric ganglia; V7, part 
(2) the lateral cords and their branches. 
of Visceral nervous system. 
Passing from a simple form like the Mail-Shell to those which 
are more specialized, we shall find that as we ascend the scale 
to higher and higher types the nervous system becomes more 
and more centralized, in the same sort of way 
as in Arthropods. The nerve-cells are no 
longer scattered throughout the central ner- 
vous system, but are collected into definite 
ganglia, of which the most important are 
thickenings of the nerve-ring. This is very 
well seen in Snails and Slugs (Gastropoda), 
a vast number of which present a similar 
arrangement to that represented in fig. 1021 
for the River-Snail (Padvadina). In the middle 
of the figure is seen the nerve-ring, which is 
thickened into three distinct pairs of ganglia 
—(1) brain-ganglia above, (2) side - ganglia 
laterally (dotted in the figure), and (3) foot- 
ganglia below. The brain-ganglia, as shown 
Fig. t021.—Central Nervous 
System of a River-Snail (Palu- 
dina), enlarged. See text. The 
circles shaded in the centre and 
connected with the pedal ganglia 
are the so-called ‘‘ears” (OZo- 
cysts) 
at the top of the figure, give origin to a cord that supplies the 
pharynx, and swells into a pair of small ganglia from which nerves 
run to the pharynx. This is part of the visceral nervous system, 
the rest of it consisting of a nerve-loop by means of which the 
VoL. IV. 
96 
