MORPHOLOGY OF ANIMALS 

 A 



B 



Bstr 



Fig. 23. A, Nervous System of Crayfish, consisting of cerebral ganglion (eg), circumoesoph- 

 ageal ring (or), sub-oesophagial ganglion (so) and ventral chain (vc) of 12 ganglia. (After 

 Huxley). B, Sense Cells in Epidermis of Earthworm; E, epidermis; sz, sense cells with fiber 

 running into the ventral chain of ganglia (Bstr). (After Heidenhain). 



(4) The tubular type of nervous system is found only 

 among the chordates; here the nervous system develops 

 from an epithelial plate {^neural plate) on the dorsal sur- 

 face of the embryo, which becomes invaginated in such 

 a way as to form a longitudinal groove, the neural groove. 

 This then separates from the epithelium as a tube (Fig. 

 22, C), which in all vertebrates is enlarged at its anterior 



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