THE NERVOUS SYSTEM OP ANTEDON ROSACEUS. 293 



pass through this, and expand to form nerve sheaths around the tube 

 feet and immediately beneath the external epidermis. 



From the above descriptions it follows that the ordinary text-book 

 accounts of the Echinoderm nervous system, which mention the radial 

 nerves and the circumoral commissure, but nothing more, require very 

 considerable modification. 



We have in addition to the Crinoids four well-marked groups of 

 recent Echinoderms, the Asterids, Ophiurids, Echinids, and Holothurids. 

 Of these four there is, I think, no doubt that the Asterids must be 

 regarded as the most primitive group, while the apodous Holothurids 

 are perhaps the most modified. This primitive character of the Asterids 

 is well illustrated by their nervous system, which as we have seen 

 above, is in the form of a continuous nerve-sheath, enclosing the whole 

 body, and directly continuous with the external epidermis of which it 

 forms the deepest layer. This nerve-sheath is thickened at certain 

 places, notably along the ambulacral grooves, where it forms the five 

 radial or ambulacral nerves. Such a condition of the nervous system 

 there is very strong reason for regarding as a very primitive one. It 

 occurs in a slightly modified form in many Coelenterates ; it occurs in 

 that primitive group of Nemertines which Hubrecht proposes to call 

 Palseonemertini ; it occurs also in the young of Sagitta and in several 

 other cases. Even in Vertebrates the central nervous system really 

 remains throughout life continuous with the epidermis, for the epithe- 

 lium lining the central canal of the cord and the ventricle of the brain, 

 was originally part of the surface epidermis. 1 



The fact that the Asterid nerve system remains in this primitive 

 condition is of considerable importance from two points of view ; in 

 the first place it shows us the parent form from which the more 

 modified nervous systems of other Echinoderms must have sprung, 

 and thereby throws great light on the mutual relations of these several 

 forms ; in the second place it is of special interest in connection with 

 the subject of the present paper, as showing that the Asterids are, in 

 at any rate one extremely important respect, far more primitive than 

 the Crinoids. I propose to say a few words on each of these points. 



Starting with the Asterid nervous system it is easy to derive from 

 it, theoretically, the nervous systems of other groups. The sinking 

 down of the radial nerves in Ophiurids and Echinids may possibly be 



1 Attention has recently been directed to this point by Sedgwick in the ' Proceedings of 

 the Cambridge Philosophical Society,' vol. iv, pi. vi, 



