762 PROCEEDINGS OF THE AMERICAN ACADEMY. 



with one another, since touching one does not result in a response 

 from another. One can draw a pencil across a branch and get a con- 

 traction of the polyps only in that line, if he does not shake the branch. 

 When a branch is shaken, all polyps begin to contract, although very 

 slowly. I saw no reactions that would indicate taste as contrasted 

 with touch. Food particles on the coenosarc cause the contraction 

 of the polyps near it, the mouth and tentacles remaining expanded; 

 but clean filter paper does the same. Neither in the field nor in the 

 laboratory did I find muscular response to light. The polyps were 

 expanded night and day alike. In the laboratory, away from the 

 sunlight they lost the Zooxanthellae and became white after a week's 

 time. 



Skeleton and Axis Epithelium. 



The structure of the axis skeleton has already' been described under 

 General Structure (p. 742) . I find the axis epithelium (Plate 4, Figs. 54, 

 58, {e'th. ax.) always present and made up of two types of cells, the 

 secreting cells and the holding cells, or desmocytes. The secreting 

 cells are long and cylindrical or prismatic. Of the two ends, the one 

 directed toward the skeletal axis may be designated as axis-end and 

 the other as mesogloea-end; the former is flat, the latter tapers and 

 is more or less rounded (Fig. 54). The large feebly staining nucleus 

 is nearly in the middle, but typically somewhat closer to the axis-end 

 of the cell. The cytoplasm is vacuolated at the mesogloea-end, but 

 near the axis it is finely granular. This type of axis cell is always 

 found at the tip of a branch, where the horny rim of the axis chambers 

 is very thin; and I interpret this as a place of most active secretion. 

 In any region of the colony, except at the very tip, some of the epi- 

 thelial cells — sometimes only one, sometimes a comparatively large 

 area of them — are modified into desmocytes (Plate 4, Fig. 41, dsm'cy.). 

 These cells are broader than the secreting cells at the axis-end, and 

 relatively shorter. At the axis-end they show a prominent border 

 of striations perpendicular to the surface. These striations are due 

 to slender rod-like differentiations of the cell, which seem to be the 

 means by which the cells hold firml}- to the axis, even when, in sections 

 cut either free hand or after imbedding, the other cells are detached. 

 Where the outline of the cell is complete (dsm'cy.), a nucleus like that 

 of the secreting cells is present. Often, however, the cell has united 

 with the mesogloea so that the boundary between the two is gone, 

 and then the nucleus may have disappeared (dsm'cy'.). The axis-face 



