1897.] PLANKTON OP THE FAEKOE CHANNEL. 807 



length. The oldest specimens of this species tit present known 

 appear to be the Plymouth specimens with 9 tentacles. 



The only other forms referable to the genus at present are 

 (1) Araclmactis hrachiolata, A. Agassiz ', obviously a different 

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

 observed by Haime ^ in the ccelenteric cavity of Cerianfhus, which 

 do not quite resemble either A. alhida or A. bournei ; with these 

 latter larvae may perhaps be identical the forms discovered by 

 Joh. Miiller and described by Buseh ' from Trieste under the name 

 of Diantliea nohilis, which have been suggested by van Beneden to 

 be Cerianthidan. 



Origin of the Mesenterial Filament. 



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

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

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

 edge of the mesentery, commonly knox^n 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^, 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 endoderm-cells of the mesenterv, 

 and agrees exactly in histological detail with the ectoderm of all 

 the stomodseum 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. Tliere is a central 

 groove (often deeper than in the figure) consisting of finely granular 

 gland-cells with very strong cilia ; these cells are practicallv 

 identical with the ectoderm of the sulcus. The groove is flanked 

 "by M'ings 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 liea 

 " unconformably " upon the A^acuolated endoderm-cells. 



I venture to repeat the suggestion (due first, I believe, to von 

 Heider) that both types of filament are ectodermal downgrovvths 

 from the stomodseura 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). 

 ^ Ann. Sciences naturelles, (4) i. p. 341 (18.54). 



^ Beob. lib. Anat. u. Entwickl. einiger wirbellosen Seetbiere, p. 122: Berlin, 

 1851, 4to. 

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



