NERVOUS SYSTEM. INVERTEBRATA. 17 



the rest of the cord ; its size is mainly due to an increase 

 of the stellate cells between the neurilemma-sheaths. 



In the specimen the introvert is retracted. A red rod has 

 been placed in the mouth and black paper beneath the 

 different parts of the nervous system. 



Metalnikoff, Zeits. wiss. ZooL, Bd. Ixviii. 1900, p. 293 

 (Anat.). 



Uexkull, Zeits. Biol., N.F. Bd. xv. 1896, p. 1 (PhysioL). 



ARTHROPODA. 

 Bethe, Arch. f. ges. Physiol., Bd. Ixviii. 1897, p. 449 



(Physiol.'). 



The central nervous system agrees with that of Worms in its 

 bilateral symmetry and general plan of construction, but is 

 always entirely free from the integument. The cerebral ganglion 

 is comparatively simple in many cases, but shows a gradual 

 increase in size and complexity of structure as the cephalic sense 

 organs become more perfect and the intelligence more pro- 

 nounced. This brain development is noticeable in the increasing 

 complication of the optic ganglia and in the development of 

 glomerular condensations in the neuropile of the antennary 

 lobes very similar to those in the olfactory lobes of Vertebrates, 

 but is particularly marked by the appearance, among the higher 

 orders, of peculiar cerebral organs (fungiform bodies) whose 

 development seems to be linked in some obscure way with the 

 growth of the intelligence. The visceral system is always 

 clearly defined. Its centres of origin (oesophageal ganglia) show 

 a gradual migration from their original post-oral position to- 

 wards the cerebral ganglion, and finally fuse with it. although 

 always united by a post-oral commissure. The ventral chain 

 in all except the lowest groups shows a considerable degree of 

 concentration laterally, but varies in longitudinal concentration 

 within the widest limits from the Phyllopods with a pair of 

 ganglia to every pair of appendages, to the Brachyura, some 

 Arachnids and Insects, in which it is represented by a single 

 post-oral ganglionic mass. 



The segments! character of the nervous system is functional 

 as well as structural, for each ganglion forms an independent 

 reflex centre for the activities of its innervation area. Co- 

 ordination is mainly due to transmission of stimuli from ganglion 

 VOL. n. C 



