SUMMARY.
The most important results in the histological part of this paper relate to the structure of the retinas of the eyes of the sensory clubs.
The retina of the distal complex eye is composed of three kinds of cells: two kinds of sensory cells (the prism and pyramid cells), and the long pigment cells ([Figs. 1-9]). The prism and pyramid cells have each an axial nerve fiber in their prisms and pyramids respectively. These fibers I could, however, trace only to the neighborhood of the nuclei. But since I could trace similar fibers in the retinal cells of the simple eyes ([Fig. 16]) past the nucleus into the subretinal nerve tissue, I believe that the axial fibers in question also extend centrad as nerve fibers into the subretinal nerve tissue. Other observers also figure such fibers as extending centrad as nerve fibers. The axial fibers of the prism cells have each a dumbbell-shaped basal body at their entrance into the pigmented part of a cell. The evidence for a body of such shape in the pyramid cells was not conclusive, though a basal body for the axial fiber exists. The long pigment cells project or retract their pigment in light or darkness respectively and thus seem to serve to check the diffusion of light in the retina. I have also supposed that these cells may serve for conducting impulses to the lens, and that the latter is adjustable.
The proximal complex eye ([Fig. 13]) has only the prism cells present in its retina, and not two kinds of cells as Schewiakoff has described (see text, pp. [53], [60], [63]) for all the eyes.
The simple eyes ([Fig. 12]), two on each side of a club, four in all, also have only one kind of cells in their retinas, and each cell has a flagellum extending into the vitreous secretion of the lumen. These flagella could be traced centrad as a nerve fiber ([Figs. 12, 16]). Similarly, a nerve fiber could be traced centrad from the flagella of the epithelial cells of the clubs. Dumbbell-shaped basal bodies for the flagella of the simple eyes could also be demonstrated, but the evidence for this in the epithelial cells of the clubs was not so satisfactory.
Other points of interest are: A secretory epithelium lining the ampulla of the clubs, and a somewhat similar epithelium lining the canals of the tentacles (Figs. [7], [27, 31]); the partial origin of the “floating bodies” in the canals of the clubs and tentacles and the stomach pockets from these epithelia ([Figs. 18, 19]); two flagella to each cell of the endothelium of the ampulla and of the pedalia (Figs. [7], [17]); the peculiar nuclei in the endothelial cells of the ampulla ([Fig. 20]); the longitudinal muscles of the tentacles being completely inclosed within canals of the supporting lamella, but near the base of a tentacle becoming subectodermal. This demonstrates their ectodermal origin. In Tripedalia it is seldom that any of these muscles become enclosed as in Charybdea ([Fig. 29]).
If to the reader my results seem to embody a somewhat heterogeneous detail, he must remember that the work consists partly in corroborating and partly in supplementing the work of previous observers, and that, in general, histological detail does not usually make the most readable paper.
Biological Laboratory, Johns Hopkins Univ., May 1899.