646 A. MILNES MARSHALL. 



the ambulacral system. The histological identity is an addi- 

 tional argument, though of less weight, on the same side. 



Accepting this homology as proved, the fact that Crinoids 

 possess part of a nerve-sheath in a primitive and unmodified 

 condition is, to my mind, strong reason for viewing them as 

 descended from forms which agreed with the recent Asterids in 

 possessing a complete nerve-sheath (though possibly very 

 unlike Asterids in other respects), and I am, therefore, disposed 

 to regard the antambulacral nervous system of a Crinoid, i. e. 

 the central capsule and axial cords with their branches, as being 

 derived from the antambulacral part of the primitive nerve- 

 sheath, and not as an entirely new set of structures possessed 

 by no other Echinoderms. A certain amount of evidence can 

 be adduced in support of this view. Dr. Carpenter has shown^ 

 that in an early stage of development of Antedon the radials 

 do not enclose the radial cords, but merely form calcareous 

 plates between the cords and the integument, which later on 

 thicken and grow round, and enclose the cords completely .^ 

 In this early stage the relations of the radial cords are very 

 similar to those of the ambulacral nerves of an adult Ophiurid 

 or Echinid,^ and as the latter have certainly acquired their 

 adult condition by becoming detached from the epidermis and 

 shifting inwards, so also may the same process be supposed to 

 have occurred in the Crinoid. The subepithelial bands of the 

 Crinoid retain their primitive positions, but the delicate con- 

 nective-tissue lamella that sometimes separates them from the 

 overlying epithelium in Antedon rosace us, and is a far more 

 evident structure in Antedon Eschrichtii and in Actino- 

 metra^ probably represents the earliest stage in the process by 

 which the nerve becomes detached from the epidermis and 

 shifted inwards. Again, the external and internal plexuses of 

 Echinus, with their connecting fibres in the substance of the 



» Carpenter, ' Phil. Trans/ 1868. 



2 That this condition is a primitive one is shown by its occurrence in some 

 of the Palseocrinoids in which the axial cords often lie in grooves, and not in 

 canals in the calcareous plates (Carpenter). 



^ Of course they do not correspond to these. 



