764 PROCEEDINGS OF THE AMERICAN ACADEMY. 



them on the Bermuda reefs. In Pseudoplexaura the stimulus for the 

 change from the secreting cell to the desraocyte must be irregular; 

 it is not associated with any particular position of the polyps or with 

 any structure that would give a regular pull or strain, since the cells 

 occur sometimes in broad patches and sometimes singly; the latter 

 are completely united with the mesogloea and are therefore fully 

 formed. They remind strongly of the desmocytes described for the 

 madreporarians by Bourne ('99), but there is no trace of the membrane 

 which Bourne found between cell and axis. In their origin they also 

 differ from those described by Bourne for Heliopora, where the stimu- 

 lus for the striations occurred before the cells were in contact with the 

 axis, to which they became adjacent secondarily; for in Pseudoplex- 

 aura the first trace of the striations is in cells already touching the 

 axis. It should be noticed that in the present paper the desmocytes 

 have been shown clinging to a horny skeleton, whereas previous 

 researches have shown them only in connection with calcareous skele- 

 tons. Probably further study will show desmocytes present in a 

 large number of alcyonarian forms. 



The origin of the horny skeleton of the Gorgonacea has been the sub- 

 ject of much controversy, with which the names of von Koch, Studer, 

 and A. Schneider have been prominently associated. A. Schneider 

 ( :05) has reviewed the literature carefully, and has shown that Ehren- 

 berg, Dana, Milne-Edwards et Haime, and von Koch have main- 

 tained an ectodermal origin; while Lacaze-Duthiers, Kolliker, Studer, 

 and Heider have not found the ectoderm involved. The arguments 

 against the ectodermal origin, as summed up b^' Schneider and 

 strengthened by his researches, have to do with (1) the presence of 

 calcareous spicules within the horny skeleton, (2) the character of the 

 union between the axis and its branches, (3) the existence of extra- 

 axial horny masses in the cortex independent of epithelium, (4) the 

 increase in size of the adult axis, (5) the embryonic origin of the 

 skeleton. 



Kolliker ('65, pp. 163-167) argued in part as follows: since the axis 

 skeleton in certain forms (Mopsea) is composed exclusively of fused 

 calcareous spicules, and since these spicules are not produced by epi- 

 thelium, the skeleton is not an epithelial product. Studer ('87) 

 and A. Schneider (:05) found numerous calcareous spicules in the axis, 

 and thought the axis made up principally of them. I have found no 

 evidence of such spicules in the axis of Pseudoplexaura, though I have 

 found one or two instances of cellular matter that I conceive to have 

 been included in the axis owing to the rearrangement of the secreting 



