ASTEROID EA. 



175 



nerve trunks, the whole lying in the deeper parts of the ecto- 

 derm. It is in connexion with a diffuse ectoneural plexus found 

 throughout the ectoderm, and at the mouth with an endoneural 

 plexus. The apical or mesoneural nervous system and the 

 deep oral system have already been sufficiently described (p. 123). 



FIG. 125. Scheme of the nervous system of the arm of a starfish (after Cuenot). a wall, 

 b body-cavity of arm ; c ampulla of tube-foot ; d tube-foot ; e radial canal of water- 

 vascular system ; 1 radial portion of ectoneural central nervous system ; 2 ectoneural plexus 

 of tube-foot ; 3 ectoneural plexus of skin ; 4 Lange's nerve cords (deep oral) ; 5 mesoneural 

 plexus just beneath the longitudinal muscle. 



The structure of the ectoneural plexus and its relation to the ectoderm 

 are best seen by examining the annular (circumoral) nerve ring or the radial 

 prolongations of it. The ectoderm here is much thickened and consists 

 of elongated columnar cells with their nuclei near their outer ends ; the 

 inner ends of the majority of the cells, which may be called supporting 

 cells, taper and form a supporting tissue for a plexus of fine nerve fibres 

 and scattered ganglion cells,which are here especially conspicuous and cause 

 the thickening of the ectoderm. Some of the 

 ectoderm cells are sense cells, and their inner 

 ends do not form supporting fibres but branch 

 out and join the nervous plexus. There are 

 special aggregations of these round the ends 

 of the tube-feet. The ectoderm along these 

 central concentrations of the ectoneural sys- 

 tem is much thickened, and its cells contain 

 yellow pigment grains which give the whole 



tract in both its annular and radial por- FlG 126 ._ Astropecten aurantia- 

 tions the appearance of a yellow streak. 



At the ends of the arms, this thickened 

 tract of nervous ectoderm is continued over 

 the terminal tentacle-like process (ocular tentacle), which forms the pro 

 jecting end of the radial water- vascular trunk. 



The eye is on the oral side of the base of this tentacle ; it is coloured 

 bright red and formed by a special thickening of the radial nerve tract 



cus, end of ray with the eye Oc 

 surrounded by spicules (from 

 Claus, after Haeckel). 



