542 A MILNES MARSHALL. 



surface of the epithelium and which are in places aggregated 

 into bundles; and (4) ganglion-cells; small nucleated cells 

 connected with the nerve-fibrils. Of these structures the two 

 latter form the nervous elements, which in Asterids are directly 

 continuous with the more superficially placed columnar cells. 



In Ophiurids the radial nerves, though having the same 

 histological structure as in Asterids, are completely distinct 

 from the epidermis, and separated from it by a thick layer of 

 calcified dermis. Here also, however, branches from the radial 

 nerves can be traced into the tube feet, where they form a 

 layer immediately beneath the epidermis. Whether an epi- 

 dermic nerve-sheath or plexus is present on the ambulacral 

 surface has not, I believe, yet been demonstrated. I have 

 been led to suspect the existence of such a sheath on physio- 

 logical grounds, but have not yet seen it. 



According to Baudelot,^ in Ophioderma longicauda 

 each of the nerves to the tube feet gives off a branch, which 

 " se portait en haut et en arriere, et m'a paru se perdre dans la 

 region dorsale du bras." 



In Echinids Krohn" was the first to show that the radial 

 nerves, which, like those of Ophiurids, are separated from the 

 external epidermis by a thick layer of calcified dermis, give off 

 branches, which accompany the tube feet through the pores in 

 the ambulacral plates, and run in the substance of the tube 

 feet as far as their free ends. 



Loven^ describes the branches which accompany the tube 

 feet as spreading out on the external surface of the test to form 

 a network of fibres with numerous ganglion-cells. He figures 

 a part of this external nerve plexus in Brissopsis lyrifera, 

 and says concerning it : " On conceit que tous les rameaux 



' Baudelot, "Etudes Generales sur le Systeme Nerveux," 'Archives de 

 Zoologie Experimentale,' tome i, 1872, p. 208. 



- Krohn, " Ueber die Anatomie der Nervensystem der Echiniden und 

 Holotliurien," ' Archiv fiir Anatomie,' 1841, translated in ' Annates des Sciences 

 Naturelles,' tome xvi, 1841. 



^ Loven, " Etudes sur les Echinoidees," ' Kongl. Svenska Vetenskaps Aca. 

 demiens Handlingar,' Bandet ii, No. 7, Stockholm, 1874, p. 8, and pi. ii, figs. 

 30 and 31. 



