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 brachiolata, A. Agassiz *, obviously a different 

 species from either of the two already described ; (2) the larvae 

 observed by Haime 2 in the co3lenteric cavity of Cerianfhus, which 

 do not quite resemble either A. albida or A. bournei', with these 

 latter Iarva9 may perhaps be identical the forms discovered by 

 Joh. Miiller and described by Busch 3 from Trieste under the name 

 of Dianthea nobilis, which have been suggested by van Beneden to 

 be Cerianthidan. 



OllIGIN OF THE MESENTER1AL ElLAMENT. 



A study of the developing mesenteries of A. albida has confirmed 

 me in the belief, advocated elsewhere by myself and by others 

 before me on histological grounds, that the thickening at the free 

 edge of the mesentery, commonly known as the mesenterial fila- 

 ment, is ectodermal in origin. The mesenteries in Cerianthidce, as 

 has long been known from the researches of A. von Heider 4 , 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 Gr of my 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 endoderrn-cells of the mesentery, 

 and agrees exactly in histological detail with the ectoderm of all 

 the stomodaeum except that of the sulcus, which has small nemato- 

 cysts, 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 

 " unconformably " upon the vacuolated endoderrn-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. Boat. Soe. N. H. vii. p. 525 (1863). 



2 Ann. Sciences naturelles, (4) i. p. 341 (1854). 



3 Beob. lib. Anat. u. Entwickl. einiger wirbellosen Seethiere, p. 122 : Berlin, 

 1851, 4to. 



4 Sitzungsber. d. k.-k, Akad. Wiss. Wien, Ixxix. (Math.-nat. 01.) p. 204. 



