294 PROFESSOR MARSHALL. 



connected with the development of the protective calcareous plates on 

 the ambulacral surface, while the similar position they hold in Holo- 

 thurids is probably due to the descent of this group from mailed 

 ancestors provided with calcareous ambulacral plates, a line of descent 

 for which there is a considerable amount of evidence forthcoming. 

 That the radial nerves of Ophiurids, Echinids, and Holothurids are 

 really the same things as the radial thickenings of the nerve-sheath in 

 Asterids, in spite of their difference of position, is practically proved 

 by the identical relations of the branches of these nerves or thickenings 

 to the tube feet, which branches in all cases alike form sheaths im- 

 mediately beneath the epidermis. The external plexus of Echinids 

 may clearly be viewed as a somewhat modified nerve-sheath ; and the 

 internal plexus of Eomanes and Ewart, which is said to be connected 

 directly with the external plexus through the substance of the test, 

 may be explained as due to this nerve-sheath having commenced to 

 shift inwards, just as the radial nerves have done, but at present 

 remaining entangled in the substance of the calcified dermis. 



As regards the origin of the Crinoid nervous system, I think that 

 the Asterid again gives us an important clue, though much yet remains 

 to be explained. It is commonly assumed that the subepithelial bands 

 of the Crinoid are homologous with the radial nerve-bands of an 

 Asterid, and I think the homology must be accepted when we consider 

 how absolutely identical the relations of these two structures are to 

 what is perhaps the most characteristic feature in an Echinoderm, i.e. 

 the ambulacral system. The histological identity is an additional 

 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 primi- 

 tive 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 1 

 that in an early stage of development of Antedon the radials do not 



1 Carpenter, ' Phil, Trans.,' 1868. 



