THE NERVOUS SYSTEM OF ANTEDON B.OSACEUS. 291 



tube feet. 1 The nervous layer can also be recognised with little 

 difficulty in the epidermis of the dorsal or antambulacral surface, and 

 Harnann 2 has shown that it really forms a continuous sheath over the 

 whole dorsal surface of the animal, which, though exceedingly thin 

 over the greater part of the back, thickens considerably at certain 

 places, notably at the bases of the respiratory processes. Hamann 

 describes the epidermis of Echinoderms as consisting of elements of 

 four kinds — (1) supporting cells, columnar cells whose deeper ends 

 are produced into fibres which pass down into the underlying dermis; 

 (2) sensory cells, columnar and sometimes ciliated cells whose deeper 

 ends are continuous with (3) the nerve-fibrils, delicate bands whose 

 direction is mainly parallel to the 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 quite 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 fomi a layer immediately beneath the epidermis. 

 Whether an epidermic 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 physiological 

 grounds, but have not yet seen it. 



According to Baudelot, 3 in Opliioderma longicauda each of the nerves 

 to the tube feet gives off a branch, which "seportait en haut et en 

 arriere, et m'a paru se perdre dans la region dorsale du bras." 



In Echinids Krohn 4 was the first to show that the radial nerves, 

 which, like those of Ophiurids, are separated from the external epider- 

 mis 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. 



1 1 believe Greef was the first to show this in 1871. 



2 Hamann, "BeitragezurHistologieder Echinodermen," 'Zeitschriftfiir wissenschaftliche 

 Zoologie,' Bd. xxxix, 1883. 



3 Baudelot, "Etudes Generates sur le Systems Nerveux," Archives de Zoologie Experi- 

 mentale,' tome i, 1872, p. 208. 



1 Krohn, " Ueber die Anatomie der Nervensystem der Echiniden und Holothurien," 

 ' Archiv fiir Anatomie,' 1841, translated in ' Annales des Sciences Naturelles,' tome xyi, 1841, 



