Proceedings of the American Academy of Arts and Sciences. VOL. XLVIII. No. 20.- MAY, 1913. CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE MUSEUM OF COMPARATIVE ZOÖLOGY AT HARVARD COLLEGE.- No. 236. THE STRUCTURE OF THE GORGONIAN CORAL PSEUDOPLEXAURA CRASSA WRIGHT AND STUDER. BY WAYLAND M. CHESTER. THE MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD COLLEGE.- No. 236. THE STRUCTURE OF THE GORGONIAN CORAL BY WAYLAND M. CHESTER. Presented by E. L. Mark, March 12, 1913. Received April 5, 1913. PSEUDOPLEXAURA CRASSA is found on the reefs of Florida, of the West Indies, and of the Bermuda Islands. It is very abundant in the shallow water of the inner reefs of Bermuda, and is there one of the two or three very common sea whips; but it is found in the deeper waters of the outer reefs as well. The range in depth, to include the greater number of colonies, is from a position near the surface at low water to seven or eight meters. Ellis and Solander (1786) described this colony under the name of Gorgonia crassa. Kölliker (1872) placed under the name of Plexaura branched, sea-rod forms in which the polyps completely retract into a comparatively thick coenenchyma, in which club-shaped and spiny spindle-shaped spicules appear. The different species were divided into two groups: Plexaura durae and Plexaura molles. Hargitt and Rogers (:01, p. 285) follow Verrill ('65, p. 34) in describing this form as Plexaura crassa. Wright and Studer ('89, p. 141-143), from observations of Bermuda specimens, created for this species a new 1 Contributions from the Bermuda Biological Station for Research. No. 27. genus, Pseudoplexaura. The new genus is characterized by them as follows: "axis horny, with a central calcareous portion, the outer layer of coenenchyme is soft and when dry friable; the inner layer contains a number of light purple or violet coloured irregularly stellate spicules or spindles with few rays." It is to be distinguished from Plexaura, in addition, by the following features, among others: colony feebly branched, older portions of horny axis solid, younger portions with calcareous particles in the center; polyps placed close together in an irregular spiral, completely retractile tentacles without spicules or having a circlet of them at their base; spicules mostly spiny spindles, with numerous pink stellate forms and a few club-shaped with attenuated foliaceous expansions. The important characters of the colony are: the relative smallness of the spicules; spicules in the outer cortex, and irregularly stellate forms in the inner cortex; the massing of the latter to such an extent as to make the inner cortex firmer when dried, while the outer is friable; the absence of spicules in the tentacles and polyps; the sluggish but complete retraction of the polyps within the cortex; and the smooth cortex surface without projecting calyces in the contracted or dried colony. The polyps are numerous. When they are completely expanded the tentacles of adjacent polyps overlap, and the coenenchyme is hidden. Each tentacle has ten to twelve pairs of pinnae. Of the three groups of alcyonarian corals, Alcyonacea, Pennatulacea and Gorgonacea,- only representatives of the first and second have had their minute structure studied recently; the Gorgonacea, to which Pseudoplexaura belongs, have received little attention except from von Koch ('87) in his very important but early comparative study. Studies on the Alcyonacea have been relatively numerous. Von Koch ('82a) described briefly the structure of Clavularia and other alcyonacean forms. Bourne ('95) described Heliopora coerulea and later made a very complete study of the origin and structure of its skeleton ('99). Ashworth ('99) studied the minute structure of Xenia Hicksonii Ash. and Heteroxenia elizabethae Köll. He found gland cells in the stomodaeum and correlated their presence there with the absence of the ventral and lateral mesenterial filaments. Hickson ('95) has given a detailed account of the cell structure of Aleyonium digitatum, and Pratt (:05) has described the digesting and mesogloea cells in several members of the Alcyonidae. She found a relatively large number of granular gland cells in the stomodaeum of feeding colonies and very few or none in starved ones. She held the mesogloeal network of cells to be neuro-phagocytic in function. By feeding with colored material, she proved the ingestion and the carriage of such material by the amoeboid movements of the mesogloeal cells. Kassianow (:08) reviewed the literature for the muscle and nerve systems in Alcyonaria digitatum, studied these systems and described in detail the cells of the ectoderm, endoderm and mesogloea with reference to them. He denied a nervous function for the neuro-mesogloeal cells of Pratt. Among the Pennatulacea, studies have been made by Korotneff ('87) and by Bujor (:01) on Veretillum. They described the cells of the ectoderm and endoderm carefully. The only complete study of the cell structure of representatives of the group Gorgonacea is by von Koch ('87), who made a comparative study of the structure and minute anatomy of the forms found in the Bay of Naples, giving most attention to Unicella (Gorgonia) cavolinii. Wilson ('84) studied the mesenterial filaments of a number of species representing the three groups. He described the difference in structure of the ventral and dorsal mesenterial filaments and the origin of each from different germ layers. Bourne ('99), in a paper giving the result of his study of the origin of the skeleton in Anthozoa, describes the origin and minute structure of the alcyonarian spicule and the structure of the massive skeleton of the alcyonarian Heliopora. He further studied the structure and origin of "holdfasts," or desmocytes, in Heliopora, as well as in madreporarian forms. Woodland (:05) reviewed the literature on the origin of the alcyonarian spicule and made a very complete study of it for Alcyonium. The names of von Koch ('78, '82b), Studer ('87, :06), and Alfred Schneider (:05) are important in the history of researches on the origin of the horny skeleton. Kinoshita (:10) has seen the origin of axis epithelium in young forms of Anthoplexaura and has confirmed von Koch's account of its ectodermal origin in the young form. The study of this gorgonian coral (Pseudoplexaura crassa) was pursued during the summers of 1909 and 1910 at the Bermuda Biological Station for Research, and during the winter of 1909-1910 at the Zoological Laboratory of Harvard University. I wish to express my great indebtedness to Dr. E. L. Mark, the Director of these Laboratories for guidance and generous assistance. |