[h] It may be objected that my criterion, the presence of axial fibers, is not necessarily characteristic of visual cells. However, the great general occurrence of such axial fibers (Patten,[5] Grenacher,[16] Schreiner,[12] Hesse,[13] myself, in simple complex eye, see below, and perhaps others) in eyes in which the retina has only one kind of cells, would seem to indicate that they are quite characteristic of visual cells. Note again that in the proximal eye of Charybdea there is only one kind of cells and with axial fibers. [i] Mr. J. C. Olsen, of the Chemical Laboratory, kindly made these tests for me. LITERATURE. LITERATURE REFERRED TO IN THE SECTION ON PHYSIOLOGY. I. ROMANES, G. J. a. ’75, ’77. The Locomotor System of Medusæ. Philosophical Transactions. London. Vol. CLXVI, pt. 1. Vol. CLXVII, pt. 2. b. ’85. Jelly-fish, Star-fish and Sea-urchins. London. II. MURBACH, LOUIS. ’95. Preliminary Notes on the Life-history of Gonionemus. Journal of Morphology. Vol. XI. III. HESSE, R. ’95. Über das Nervensystem und die Sinnesorgane v. Rhizostoma Cuvieri. Zeit. Wis. Zool., B. LX. IV. EIMER, TH. Zoologische Untersuchungen. ’74. Würzburg Verhandlungen. VI. Bd. V. HAECKEL, E. ’79. Monographie der Medusen. Jena. VI. BERGER, E. W. ’98. Abstract of Dr. F. S. Conant’s Notes on the Physiology of the Medusæ. Johns Hopkins University Circulars. Vol. XVIII, No. 137. VII. (See also 8, below.) LITERATURE REFERRED TO IN THE SECTION ON HISTOLOGY. 1. CARRIÈRE, J. ’85. Die Schorgane der Thiere. München u. Leipzig. 2. HAAKE, W. ’87. Scyphomedusen des St. Vincent Golfes. Jen. Zeit. f. Naturwis., Bd. XX., pp. 596-597, 602-604. 3. CLAUS, C. ’78. Über Charybdea marsupialis. Arb. aus dem Zool., Inst. Univers. Wien., Bd. I. 4. SCHEWIAKOFF, W. ’89. Beiträge zur Kenntniss des Acalephenauges. Morph. Jahrb., Bd. XV, H. 1. 5. PATTEN, WILLIAM. a. ’89. Studies on the eyes of Arthropods. II. Eyes of Acilius. Journal of Morphology. Vol. II. b. ’98. A Basis for a Theory of Color Vision. American Naturalist. Vol. XXXII, No. 383. 6. APATHY, ST. ’97. Das Leitende Element des Nervensystems u. seine topographischen Beziehungen zu den Zellen. Mitt. Zool. Stat. Neapel., Bd. XII, H. 4. 7. PARKER, G. H. ’97. Photomechanical Changes in the Retinal Pigment Cells of Palæmonites, and their Relation to the Central Nervous System. Bull. Mus. Comp. Zool. Harvard Coll. Vol. XXX, No. 6. 8. CONANT, F. S. a. ’97. Notes on the Cubomedusæ. Johns Hopkins University Circulars. Vol. XVII, No. 132. b. ’98. The Cubomedusæ. Memoirs Biological Laboratory Johns Hopkins Univ. Vol. IV, No. 1. 9. A REVIEW OF 5b. ’99. A Theory of Color Vision. Natural Science. Vol. XIV, No. 85. 10. HERRICK, F. H. ’91. The Embryology and Metamorphosis of the Macroura (Brooks and Herrick). Natl. Acad. Sciences. Vol. V, p. 454. 11. HERTWIG, O. & R. ’78. Das Nervensystem und die Sinnesorgane der Medusen. Leipzig.