■and Anatomy of the Echinodermata. 367 



The latter are always smaller and generally possess a basal 

 cell-process, a direct continuation of the cell-substance, which 

 shows a different refractive power from the nerve-iibres, and 

 therefore, if only on that account, has nothing to do with 

 them, and, further, is much stronger and has a greater 

 diameter. 



The ganglion-cells, as they occur in the peripheral parts 

 of the nervous system, are of two different forms. If they 

 lie within the nerve-fibres, the nerves of the skin (I am 

 referring to nerve-cords), they have the same form as in the 

 main trunks. Besides this kind cells occur which are charac- 

 terized by their size, their large pale nuclei, and their con- 

 stantly distinct nucleoli. These lie peripherally upon the 

 nerve-cords, and where nerve-fibres issue from the nerve- 

 cords, for example to run to the muscular fibres (in the pedi- 

 cellariee the ramifications between the muse, adductores, in 

 the basal annular nerve of the spines of Splicer echinus ^ Echinus^ 

 CentrostephanuSj &c.), form a coating between the muscular 

 fibres embraced by the nerve-fibres. These cells measure 

 about 0*07 millim.^ and their circular nuclei 0"002-0"003 

 millini. That they differ widely from the cells of the 

 covering-epithelium there can be no doubt. 



If I have discussed the question of the interpretation of 

 these coating-cells in a detail which may appear superfluous 

 to many, this is due to the wish to render my description as 

 conclusive as possible. 



If we are to derive the Echinida directly from the Star- 

 fishes, we must seek in them for organs homologous with the 

 tentacle and eye-spots. As is well known there ai'e upon the 

 intergenital plates (ocellar plates) in many Sea-urchins pig- 

 ment-spots which it has been supposed might be interpreted 

 as eyes, seeing that they are situated in spots homologous 

 with the ends of the arms of Starfish. As I have already 

 shown, we have to do here by no means with structures 

 resembling the eye-spots of Starfishes, but only with accumu- 

 lations of pigment which may sometimes be present, some- 

 times absent. But that we may in this case with some justice 

 speak of degenerations of the eye-spots appears from the 

 presence of a tentacle, although a modified one, in the Echi- 

 nida *. The tentacle pierces the intergenital plate, and thus 

 comes to lie partly in and partly upon the latter. A water- 

 vessel (ambulacral) and a nerve-trunk terminate in it in the 

 same way as in the Starfishes. Nay, even mobility cannot 

 be wholly denied to the Echinidan tentacle, seeing that it, or 



• See Hamann, " Vorl. Mitth. zur Morphol. der Echiniden, No. 5," in 

 Sitzungsb. der med.-naturw. Gesellsehaft zu Jena, 1886, Heft 2. 



