The general structure of the chief central nerve trunks is quite similar as shown in sections. Figs. 6, 7 and 8. The nerve trunks have about one to two layers of cells, the main part of the nerves are composed of longitudinal fibers. There are not so many evident vertical fibers from cells as found in starfish and some other forms. This change in position of the fibers may be in part due to the general modification of structure. Whether this arrangement leads to other types of nerve association is a question. When the nerve trunks are removed, stained in methylene blue and examined with the microscope something of the arrangement of the cells may be seen. In the circumoral and oral radial nerves the nerve cells are thickly massed from side to side, but in the upper part of the aboral nerve there is an evident arrangement of nerve cells in zones. There is usually a central more or less clear zone, next on each side a rather dense cell area and next on each side a very dense cell area, then a narrow nearly clear zone on each side again. As a rule slightly larger cells are found near the nerve trunks and as some of these seem to send long branches out into the lateral trunks, they may be motor or sensory, the association neurones are probably the smaller cells in farther. The cells seem multipolar in most cases and in fact much more modified than the cells of starfish or sea-urchin. Figs. 9 and 10. REFERENCES _Delage and Herouard_ 1903 Traite de zoologie concrete. T. iii. Les Echinoderms. _Hamann, O._ 1887 Beitrage zur Histologie der Echinodermen. Jenna Zeit. Nat. W. xxi. _Hilton, W. A._ 1917 Some remarks on the nervous system of two sea-urchins. Jour. ent. and zoo. vol. ix, no. 4. (_Contribution from the Zoological Laboratory of Pomona College_) Explanation of Figures Fig. 1. Diagram of one fifth of Aristotle's lantern of _Dendraster_ showing three loops of the circumoral nerve ring, and parts of three radial nerves, the central one partly hidden at its origin by the lantern. The nerves are in black. x9. Fig. 2. Drawing of part of the first part of an oral radial nerve. x9. Fig. 3. Drawing of the lower end of an oral radial nerve. x9. Fig. 4. Drawing of the upper part of an aboral radial nerve. The eye spot region is up in the figure. x9. Fig. 5. Camera lucida drawing of a part of an aboral nerve showing position of cell areas. x70. Fig. 6. Drawing of a section of an oral radial nerve. x300. Fig. 7. Drawing of a section of circumoral nerve. x300. Fig. 8. Drawing of a section of aboral nerve. x300. Fig. 9. Nerve cells from central regions of a radial nerve. The arrangement is as shown in the drawing, cells of various levels shown as one layer. Some of the processes possibly relate nearby cells, but most fibers run into the general fibrous mass. All fibers or fibrils are small. There is some indication of tigroid substance in some of the cells. x450.