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Show 1897.] PLANKTON OF THE FAEROE CHANNEL. 807 length. The oldest specimens of this species at present known appear to be the Plymouth specimens with 9 tentacles. The only other forms referable to the genus at present are (1) Arachnactis brachlolata, A. Agassiz1, obviously a different species from either of the two already described ; (2) the larvae observed by Haime 2 in the ccelenteric cavity of Cerianfhus, which do not quite resemble either A. albida or A. hournei; with these latter larvae may perhaps be identical the forms discovered by Joh. Miiller and described by Busch 3 from Trieste under the name of Dianthea nohills, which have been suggested by van Beneden to be Cerianthidan. ORIGIN OF THE MESENTERIAL FILAMENT. A study of the developing mesenteries of A. albida has confirmed me in the belief, advocated elsewhere by myself and by others before m e on histological grounds, that the thickening at the free edge of the mesentery, commonly known as the mesenterial filament, is ectodermal in origin. The mesenteries in Cerlanthldce, as has long been known from the researches of A. von Heider4, are of two kinds-fertile (generative) and digestive, which generally alternate one with another, and, as he mentions very briefly, carry two different kinds of filaments, which become differentiated about stage G of m y specimens. The filament of a digestive mesentery (fig. 2) is of a type familiar to all students of Anthozoa : it consists of densely packed gland-cells of at least two kinds, among which lie nematocysts in all stages of development; this tissue abuts, quite sharply and without transition, on the undoubtedly endoderm-cells of the mesenterv, and agrees exactly in histological detail with the ectoderm of all the stomodaeum except that of the sulcus, which has small nematocysts, if any. The filament of a fertile mesentery (fig. 3) is different from the foregoing both in shape and in histological detail. There is a central groove (often deeper than in the figure) consisting of finely granular gland-cells with very strong cilia ; these cells are practically identical with the ectoderm of the sulcus. The groove is flanked by wings containing large gland-cells and nematocysts ; next to these come three sets of simpler cells, the nuclei of the first and third set staining very strongly. The last of these three sets lies " unconformabl)r " upon the vacuolated endoderm-cells. I venture to repeat the suggestion (due first, I believe, to von Heider) that both types of filament are ectodermal downgrowths from the stomodaeum along the free edge of the mesentery, on the following grounds:- 1. The histological structure of the chief part of both filaments is . 1 Journ. Bost. Soc. N. H. vii. p. 525 (1863). 2 Ann. Sciences naturelles, (4) i. p. 341 (1854). 3 Beob. iib. Anat. u. Entwickl. einiger wirbellosen Seethiere, p. 122: Berlin, 1851, 4to. 4 Sitzungsber. d. k.-k, Akad. Wiss. Wien, lxxix. (Math.-nat. CI.) p. 204. |