292 PROFESSOR MARSHALL. 



Loven 1 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 

 congoit que tous les rameaux du tronc nerveux se divisant de cette 

 maniere, il y aura, r^pandu a la surface du corps, un systeme nerveux 

 peripherique extr&menient developpe, fournissant des nerfs aux radioles, 

 aux pedicellaires, aux clavicles des fascioles, et en general a toutes les 

 parties externes." 



Fredericq 2 was led, from a series of experiments on Echinus and 

 Toxopneustes, to suspect the presence " d'un plexus nerveux situe 

 dans l'epaisseur de la peau qui recouvre le test a l'exterieur," but did 

 not succeed in demonstrating its existence anatomically. 



More recently Romanes and Ewart 3 have described experiments on 

 living Echini, which lead them to believe in the existence not only 

 of an external nerve plexus outside the test, but also of an internal 

 plexus on its inner surface ; they further believe that the two systems 

 are connected by nerve-fibres running through the plates of the test. The 

 external plexus they figure 4 and describe "as lying almost immediately 

 under the surface epithelium, and extending from the shell to the 

 spines and pedicellarise ;" and in a postscript they state that they 

 " have been successful in obtaining full histological demonstration of 

 the internal nervous plexus of Echinus," and promise full descriptions 

 of " its character, distribution, and mode of communication with the 

 external plexus." 5 



Concerning Holothurids, both Krohn and Baudelot describe, in the 

 memoirs cited above, branches from the radial nerves to the tube feet. 

 More recently Hamann, 6 in the paper already quoted, has added 

 valuable details concerning the distribution of these branches. He 

 shows that the branches to the tube feet, which are at first situated, 

 like the radial nerves from which they arise, beneath the dermis, soon 



1 Loven, " Etudes sur les EchinoidSes," ' Kongl. Svenska Vetenskaps Acadeiniens Hand- 

 lingar,' Bandet ii, No. 7, Stockholm, 1874, p. 8, and pi. ii, figs. 30 and 31. 



2 Fredericq, "Contributions al'etude des Echinides," 'Archives de Zoologie Experiment 

 tale,' tome v, 1876, p. 438. 



3 Romanes and Ewart, "On the Locomotor System of Echinodermata," 'Phil. Trans.,' 

 1881, part iii. 



1 Romanes and Ewart, loc. cit., p. 836, pi. 80, figs. 16—18. These figures are very different 

 to Loven's, which, however, were drawn from another genus. 

 6 Loc. cit., p. 882. 

 e Hamann, loc. cit., p. 168, and pi. ii, figs. 51, 52, 53. 



