290 PROFESSOR MARSHALL. 



connection with the results detailed above, and I propose in this 

 concluding section to notice briefly a few of the more important of these. 



In the first place the morphological difficulty arising from the 

 possession by Antedon of an antambulacral in addition to the typical 

 ambulacral nervous system of Echinoderms must be considered. This 

 objection has been strongly urged by Ludwig, and constitutes indeed 

 the real ground of his dissent from Dr. Carpenter's views ; and it must 

 be admitted that the presence of a complicated nervous system in 

 Crinoids, which is apparently altogether unrepresented in other Echi- 

 noderms, is a feature which a morphologist might well shrink from 

 accepting until the fullest proof was forthcoming. This proof I have 

 attempted to supply in the preceding section ; the morphological 

 puzzle, however, still remains to be considered. 



Tiedemann 1 was the first to describe the ambulacral nervous system 

 of Echinoderms, and since his time the five radial bands with their 

 connecting circumoral commissure have been universally accepted as 

 constituting the typical Echinoderm nervous system. This nervous 

 system, as was pointed out by Tiedemann, is differently situated in 

 the different groups : 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 2 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 presence and the nervous 

 nature of these radial bands and oral commissure, and adding 

 much to our knowledge of their minute structure and relations, 8 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 



1 Tiedemann, ' Beobachtungen ueber das Nervensystem und die sensiblen Erscheinungen 

 der Seesterne ;' Meckel's 'Archiv. fiir Physiologic,' Bd. i, 1815; and ' Anatomie des Rohren- 

 holothuries, des Seesterns und Steineigels,' Landshut, 1816. 



a Agassiz et Desor, " Catalogue raisonne' des families des Echinodermes," 'Annales des 

 Sciences Naturelles,' 1846. 



3 Ludwig's researches on the nervous system of Echinoderms, embodied in his 'Morpho- 

 logische 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. 



