Memoirs: The Structure of Xenia Hicksoni, nov. sp., with some Observations on Heteroxenia Elizabethæ, Kölliker

The following is a recapitulation of the new points to which reference is made (paragraphs 1, 2, and 6 have been already published in ‘Proc. Roy. Soc.,’ 1898): 1. The absence of ventral and lateral mesenterial filaments usually present in the polyps of the Alcyonaria. The only previously recorded examples of the absence of these filaments are the siphonozooids which occur in some other Alcyonacea and in Pennatulids. 2. The absence of these filaments is correlated with the presence of gland cells in the stomodaeum, which occur especially in the ventro-lateral walls which abut on the siphonoglyph. Their position is suggestive of the digestive function of the cells, as their secretion can be readily poured out on to the ingoing food particles. 3. The non-retractile nature of the bodies of the polyps and of the stems is accounted for by the absence of musclefibres from their ectoderm cells, and by the presence of numerous spicules in these parts. The presence of ectodermic muscles in the tentacles, pinnules, and distal millimetre of the bodies of the polyps, together with the absence of spicules from these parts, confers the power of contractility, slight though it is, upon these parts. The muscle processes of the ectoderm cells (where present) are longitudinal in direction, while those of the endoderm cells are circular (except the protractor and retractor muscles on the mesenteries). 4. Nematocysts were found in Sarcophyton. 5. An extraordinary number of spicules is present in the basal part of the colony, and much of the mesoglœa is converted into a dense horny substance. Thus a firm base of attachment is provided which would afford a rigid support for the branches which arise from it. 6. Many of the endoderm cells lining the cœlentera and tentacles bear giant flagella, which may attain 120 µ in length. 7. In adult polyps the primitive genital cells are formed by differentiation of some of the endoderm cells which cover the mesenteries. These genital cells migrate into the mesoglœa of the mesenteries, and then move outwards, one at a time, each cell pushing the endoderm and a thin film of mesoglœa before it, and so forming a small tubercle on the side or end of the mesentery. By division of the genital cell the spermatozoa are produced. They remain until ripe, surrounded by a thin film of mesoglœa, and by a layer of endoderm cells, many of which contain from two to five nuclei. 8. The longitudinal canals which traverse the mesoglœa of the stem are very well developed, and are physiologically, and possibly morphologically, equivalent to the cœnenchymal tubes of Heliopora cœrulea. 9. The nervous system is similar to that of Alcyonium; the stellate cells immediately outside the endodermic musclefibres are very clearly seen. 10. A complete series of developing polyps is described, beginning with very young specimens, in which the tentacles are devoid of pinnules, and ending with the adult polyps with multi-pinnuled tentacles. 11. A similar series of polyps of Heteroxenia Elizabethae is described and compared with the siphonozooids of the same colony. It is concluded that Heteroxenia is undoubtedly dimorphic, as stated by Kolliker and Bourne. 12. The origin of the buds of Xenia Hicksoni from the superficial canal system, their subsequent growth, and the appearance in them of the mouth, stomodaeum, siphonoglyph, gland cells in stomodaeum, mesenteries, dorsal mesenterial filaments, and flagella of the endoderm cells, are traced. It is remarkable that the sexual cells are already differentiated in a young polyp 95 mm. long.