ON THE LOCOMOTOR SYSTEM OF ECHINODERMATA. 
877 
Literature. 
For a general account of the literature on the morphology of the Echinoderms up 
to 1872 reference had best be made to Baudelot in 'Archives de Zoologie Exp. et- 
Gen.,’ t. i., pp. 176-216. 
The observer who first detected the nervous ring in Echinus was Van Beneden, 
who published his description in ‘ L’Echo du Monde Savant’ in 1835. A more 
detailed description was afterwards given by Krohn in Muller’s ‘Archives’ in 1841, 
which was followed by the well-known investigations of Valentin", L. and A. 
Agassiz f, J. Muller];, Hoffmann§, and Loven||. More recently still a valuable 
paper has been published by Fredericq If, of the existence of which we were not 
aware until our own paper had been written. As we now find that some of our 
results have been anticipated by this author, we shall here devote a few paragraphs 
to epitomising all the more important features of his work. 
Fredericq found that the pentagonal nerve-ring of Echinus and its five radial 
nerves are all contained in as many sheaths or tubes of membrane, which are mesen- 
tery-like expansions of the lining membrane of the shell. These enveloping tubes 
send out lateral branches which contain the lateral nervous offshoots ; the latter pass 
out of the ambulacral pores in company with the pedicels which they serve to enervate, 
a delicate nerve running along the whole length of each pedicel to terminate at its 
distal end in a tactile organ. 
Fredericq considers it probable that in their passage through the ambulacral pores 
the nerves also send branches to the spines and pedicellarise ; these branches, however, 
he failed to detect. The radial or ambulacral nerve-trunks terminate in the ocular 
plates. The latter, however, show no histological evidence of supporting any structure 
resembling an ocular apparatus; and Fredericq could obtain no physiological evidence 
of sensibility either to solar or to artificial light. He does not state clearly what his 
experiments in this connexion were, and so we infer that they cannot have been the 
same as ours. He regards the pigment spot as a “ fiction.” 
The nerve-ring sends off, in addition to the ambulacral trunks, the nerve-cords to 
the intestine. In the ring and trunks there is no differentiation into ganglia and 
fibres, but the whole is in structure uniform and in function central. The brown 
colour is due to elongated and irregular cells having conspicuous nuclei filled with 
pigment, and supposed to betoken connective tissue. The nervous tracts are them- 
* ‘ Anatomie du genre Echinus,’ 1841. 
| ‘Bulletin of tlie Museum of Comp. Zool.,’ in vols. i., ii., and iii., “Revision of the Echini;” and 
‘ American Naturalist,’ vol. vii., pp. 398-406. 
J ‘ AbLandl. der Konigl. Akad. der Wissens. zu Berlin,’ 1853, and Muller’s ‘ Archiv.,’ 1853, p. 175, 
and 1850, p. 127. 
§ ‘ Nederliindisches ‘Archiv. fur Zool.,’ i., 1871, p. 54. 
|| ‘ Anns, and Mag. Nat. Hist.,’ 1872, p. 28, and ‘ Etudes sur les Echinoidees,’ 1875. 
‘ Arch, de Zool. Exp.,’ t. v., pp. 429-440. 
