536 Dynamic Theory. 



sucker. These last may act by reflex stimulation independently of the 

 central ganglion, or they may be stimulated by the central ganglion, and 

 so act in harmony with the rest of the body. The central ganglion is 

 the nervous center of the special sense organs, sight, &c. , so that when 



FIG. 259. Nervous system of Pond Muscle 

 (Anodonta). 

 O.-Mouth. 



Og. Cerebral ganglion. 

 <S'e. Labial palps touch organs. 

 G. Generative gland. 

 OK. External opening of kidney. 

 OCT. Opening of generative gland. 

 K. Gills ( large-paired organs). 

 A. Anus. 



Vg. Splanchnic ganglion. 

 Ik P. Foot. 



Pg. -Foot ganglion. (After Keber.) 



the action is governed by it, the stim- 

 uli of sight, and other senses, over- 

 rule the stimulus of touch in the 

 sucker. The suckers of the Cuttle 

 fish will act by reflex stimulation when 

 they are cut off from the rest of the 

 body, it only being required to place 

 them in contact with some object. 



Among the articulates 

 there is a central ganglion 

 at the head end, the stim- 



FIG. 260. Segmented worm. 

 Centipede, showing nervous sys- 

 tem. 



^Byj 



% 



FiG. 259. 



ulation of which is derived from the specialized senses, as 

 in the Cephalopods, and there are likewise independent cen- 

 ters placed in each of the segments of the body. These 

 segments, however, are not arranged around a center, but 

 are placed in a row, one after the other. ( Fig. 260.) The 

 segmental ganglions are underneath the cavity of the stom- 

 ach, one in each segment. Each segment usually supports 

 two limbs, and the segmental ganglion is the medium of the 

 reflex action which takes place in them, the central ganglion 

 being the medium for starting action caused by stimulation 

 from the special sense organs. The cephalic, or head gang- 

 lion, is placed above the mouth, thus indicating the position 

 to be taken by the brain in the vertebrates, of which this 

 ganglion is undoubtedly the forerunner. Its most impor- 

 tant stimulating sense is, no doubt, the sense of sight. It is situated 

 near the eyes, and in size usually bears a direct proportion to them. 

 This ganglion is connected with the first segmental ganglion, which lies 

 below it on the underside of the esophagus, or gullet, by a band of 

 nervous matter on each side, so that the effect is to surround the esoph- 



f 



FIG. 260. 



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