NERVOUS SYSTEM OF ANTEDON ROSAOBUS. 541 



in Asterids it is quite superficial, while in Ophiurids, Echinids, 

 and Holothurids it is much more deeply placed, being separated 

 from the surface by a thick layer of cutis which in the two 

 former groups is firmly calcified. Agassiz^ urged this difference 

 of position as an objection to the homology of the radial 

 bands in Asterids and Echinids, but the objection was not 

 sustained. More recent researches, while confirming the pre- 

 sence and the nervous nature of these radial bands and oral 

 commissure, and adding much to our knowledge of their minute 

 structure and relations,^ have, however, tended to show that 

 they only form a part of what is really a very widely-spread 

 and diffuse nervous system. 



Thus in Asterids it is very easy to demonstrate that the 

 nerve-layer, which is perfectly continuous with the epidermis, 

 of which indeed it forms the deepest stratum, is not confined to 

 the floor of the ambulacral groove, but extends ; though as a 

 thinner layer, over the tube feet.^ The nervous layer can also 

 be recognised with little difficulty in the epidermis of the 

 dorsal or antambulacral surface, and Hamann* has shown that 

 it really forms a continuous sheath over the whole dorsal surface 

 of the animal, which though exceedingly thin over the greater 

 part of the back, thickens considerably at certain places, 

 notably at the bases of the respiratory processes. Hamann des- 

 cribes the epidermis of Echinoderms as consisting of elements of 

 four kinds — (1) supporting cells ; columnar cells, whose deeper 

 ends are produced into fibres which pass down into the under- 

 lying dermis; (2) sensory cells; columnar and sometimes ciliated 

 cells whose deeper ends are continuous with (3) the nerve- 

 fibrils ; delicate bands whose direction is mainly parallel to the 



' Agassiz et Desor, " Catalogue raisonue des families des Echiuodermes," 

 ' Annales des Scieuces Naturelles/ 1846. 



* Ludwig's researches on the nervous system of Echinoderms, embodied 

 in his ' Morphologische Studien an Echinodermen,' are of especial value 

 and importance ; and a recent paper by Hamann, referred to below, contains 

 many new points of great interest. 



^ I believe Greef was the first to show this in 1871. 



■• Hamann, " Beitrage zur Histologic der Echinodermen," ' Zeitschrift fiir 

 Wisseuschaftliche Zoologie/ Bd. xxxix, 1SS3. 



