544 A. MILNES MARSHALL. 



feet, which are at first situated, like the radial nerves from 

 which they arise, beneath the dermis, soon 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, Echi- 

 nids, and Holothurids. Of these four there is I think no 

 doubt that the Asterids must be regarded as the most primi- 

 tive 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 ambu- 

 lacral 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 Paloeonemertini ; it occurs also in the young of Sagitta and 

 in several other cases. Even in Vertebrates the central ner- 

 vous system really remains throughout life continuous with 

 the epidermis, for the epithelium lining the central canal of 

 the cord and the ventricles of the brain, was originally part of 

 the surface epidermis.^ 



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 Echi- 

 noderms must have sprung, and thereby throws great light on 

 ^ Attention has recently been directed to tliis point by Sedgwick in the 

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



