NERVOUS SYSTEMS OF INVERTEBRATES 



M. 



N.R. 



Gills 



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- 



* J Fig. 1020. A Mail-Shell (Chiton) dis- 



vous system are related to the digestive sected from above, to show central N er - 



' . vous System 



organs. The visceral nervous system con- ^ Mouth . IA ^ intestinal aperture . 



sistS in this Case Of (i) the nerVeS Which ^-^^S-, P.O., p^l cord; L.C., 



lateral cord; St., stomach nerve passing 



run from the nerve-ring to the pharynx, back to pair of gastric ganglia; K, P art 



, i . , , f Visceral nervous system. 



(2) the lateral cords and their branches. 



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 (Paludina). In the middle 

 of the figure is seen the nerve-ring, which is Fig. io 2 i.-centrai Nervous 



t . i i i r 1 System of a River-Snail (Palu- 



thickened into three distinct pairs of ganglia dina]i enlarged, see text. The 



s - called " ears " (oto ~ 



-(i) brain-ganglia above, (2) side -ganglia 

 laterally (dotted in the figure), and (3) foot- 

 ganglia below. The brain -ganglia, as shown 

 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. 



