112 THE VOYAGE OF H.M.S. CHALLENGER. 



erwithnten zu den Tentakelu sah icli nirgends von dem Nervcnstamm des Arms oder der 

 Pinnula abtreten." ^ Thus speaks Ludwig, who has given us a careful description of 

 tlie ambulacral nerves of Antedon eschrichtt, the type in which they are more completely 

 differentiated from the ectoderm than in any other Crinoid. 



Immediately beneath the closely packed ciliated epithelium which lines the food- 

 groove is the band of nerve-fibrils, which thins away somewhat in the middle line 

 above the radial blood-vessel. It is covered by a very delicate sheet of connective 

 tissue on which the epithelial layer rests, and it is traversed vertically by delicate 

 threads of a similar nature which break up the whole nerve-band into bundles of fibrils 

 with numerous minute cells intercalated among them (PI. LX. figs. 4, 6, n). Ludwdg 

 was uualile to find this connective tissue sheet in Antedon rosacea, and I have not met 

 with it in any other Crinoid but Antedon eschrichti. All the other tyj)es that I have 

 examined have a much less defined nerve-band than this species (PI. Villa, figs. 4, 5 ; 

 PI. LVII. figs. 3, 4 ; PI. LIX. figs. 1, 5 ; PI. LX. figs. 1, 2— n) ; and the vertical fibres 

 which cross it are continuous with the extended lower ends of some of the epithelial 

 cells in the layer above. In fact, both Ludwig and myself have observed this absence 

 of a basement membrane and the connection of the epithelium with the vertical fibres 

 in some individuals of Antedon eschrichti, other sections of which j)reseut the appearance 

 described above. 



Judging from Hamann's observations on the Asterids and Holothurians,^ and also 

 from those of Koehler^ on the Urchins, we may consider it certain that among these 

 " StutzzeUen " there are likewise sense-cells or neuro-epithelial cells, the inferior ends 

 of which are connected with nerve-fibrils. 



Not having w^orked with sufficiently well preserved material, I have never seen them, 

 and they escaped the notice of Ludwig both in Asterids and in Criuoids ; but I have no 

 doubt whatever as to their presence. 



As regards the ceUular elements of the Echinoderm nervous system, it is becoming 

 gradually recognised that nerve-cells of the usual well defined type are either altogether 

 absent, or confined to certain specially sensitive parts of the body. Neither Hamann nor 

 Senon have found anything but bipolar cells in the nervous system of Holothurians. 

 Similar cells have been described and figured by Koehler in the Urchins ; while, according 

 to Eomanes and Ewart*the cell protoplasm " is generally seen to project in two, or 

 sometimes in three directions." The radial nerves of the Asterids, according to Hamann, 

 contain numerous fusiform or bipolar cells, among which are a few with more than two 

 processes ; while larger cells, both bipolar and multij)olar, occur in the neighbourhood of 

 the terminal tentacle. Considering the reduced condition of the ambulacral nerve in the 



^ Crinoideen, loc. cit., p. 264. 



^ Beitrage ziu' Histologie der Echinodermen, Mitth. I., II., Zeitschr.f. kiss. ZooL, Bd. xxxix. pp. 146, 309. 



3 Op. cit, pp. 50-54, pi. vi, fig. 47. 



* Observations on the Locomotor System of Echinodermata, Phil. Trans., 1881, p. 836. 



