366 Dr. O. Hamann on the Phylogeny 



system in the Echinida the same as those of the Asterida, or 

 derivable from those of the latter group ? To decide this 

 question we maj refer briefly to the constitution of the ner- 

 vous system of the Asterida. The cerebral ring and the 

 ambulacral nerves consist of nerve-fibres intermixed with 

 ganglion- cells, which run between the processes of the un- 

 usually elongated, filiform, epithelial cells of the ambulacral 

 groove. These epithelial cells I have named " supporting- 

 cells," and their basal processes ^' supporting-fibres ; " the 

 latter are the so-called transverse fibres of older writers, 

 which run perpendicular to the nerve-fibres. In the Echi- 

 nida the central nervous system consists of the following 

 elements : — the nerve-fibres with the ganglion-cells, and, 

 applied to these, cells the nature of which may be a matter of 

 dispute. This coating of cells, which lies peripherally upon 

 the main nerve-stems and the central ring, is regarded by 

 Fr^dericq as nervous — it is supposed that we have here to do 

 with ganglion-cells which lie upon the cords of nerve-fibres 

 in the same way as is the case in many Yermes, for ex- 

 ample. 



Whether these cells have acquired the function of ganglion- 

 cells seems doubtful to me. Judging from their origin they 

 are epithelial cells which have come to lie in the mesoderm 

 together with the nerve-fibres originally {ia the ectohlast) epi- 

 thelially situated and produced. In the first place they func- 

 tion as a covering epithelium, a protective coat for the fine 

 nerve-fibres, as 1 have already shown in the Holothuriee, and 

 as seems to me to follow pretty certainly from a comparison 

 with the Asterida. 



That these cells form a covering epithelium, a protective 

 covering, appears further from their basal supporting -fibres^ 

 which traverse the nerve-fibres perpendicidarly. I^liese 

 supporting-fibres have, however, hitherto escaped the notice 

 of naturalists in the Echinida. I believe that even those 

 who are inclined to interpret the covering-epithelium as of 

 nervous nature can no longer, after the discovery of the sup- 

 porting-fibres, uphold this opinion to its full extent. But 

 what further goes against the nervous nature of these cells is 

 their difference in form and size from the true nerve-cells in 

 the main trunks, and the nerve-cells which form a peripheral 

 coating at the point of bifurcation of the main nerve-cords. 



The ganglion-cells which are situated in the main trunks 

 and the cerebral ring possess an oval nucleus, which always 

 stains of a lighter colour than the nucleus of the covering- 

 cells. A nucleolus is usually to be seen. The size of the 

 ganglion-cells is different from that of the covering-cells. 



