1897.] PLA>'KTON OF THE FAEBOE CHAXXEL. 809 



With regard to the distribution of the two types of mesenteries 

 and filaments in Arachnactis, the "directive" pair practically carry 

 no filament ; for a very few sections below the end of the sulcus 

 they have a slight thickening resembling the type of a fertile 

 mesentery, but almost immediately assume the appearance indicated 

 in fig. 4. The mesenteries next to them are of the fertile type, 

 and the next ensuing of the digestive type ; from that point 

 onwards the alternation is apparently regular : — 



Fertile : 3, 1, 4, 6, 8, &c. 1 Numbered in order of 

 Digestive: 2, 5, 7, 9, 11, &c. j successive development. 



The differentiation of the filaments of the two kinds of me- 

 senteries in the adult Cerianthus is apparently not mentioned 

 by the brothers Hertwig'; their figure 3, pi. viii., practically 

 unites the main features of my figures 2 and 3. Unfortunately, 

 the specimens of Oenanthus at my disposal are not very well 

 preserved, but even in them it is obvious that there is a differenti- 

 ation of the two filaments, of the same kind as, although not 

 precisely identical with, that which I have described above for 

 Arachnactis. Very j'^oung germ-cells are recognizable in both 

 tvpes of mesentery in the adult. 



I have seen nothing in Arachnactis of the small " directive " 

 mesenteries, not attached to the stomodseum, which are mentioned 

 by von Heider as occurring in Cei-ianthus. 



Note. — Since the MS. left my hands, I have received a letter 

 from my friend Prof. Karl Brandt of Kiel, which informs me that 

 Prof, van Beoeden has a paper in the press dealing with the 

 Arachnactis of the Plankton and other German expeditions ; this 

 will doubtless throw more light on the distribution of the various 

 species. Prof. Brandt informs me that the genus appears to have 

 been widely taken in the North Atlantic (' National ') and in the 

 North Sea (' Holsatia ' 1885, Nordsee Expedition 1895). 



EXPLANATION OP PLATE XLVII. 



Arachnactis albida, M. Sars. 



Fig. 1. Diagram showing the order of development of mesenteries, marginal 

 tentacles, and oral tentacles (p. 805). 



2. Section of the filament of a digestive mesentery, X 600 (p. 807). 



3. Section of the filament of a fertile mesentery, x600 (p. 807). 



4. Section of the thickened edge, presented both by a mesentery which haa 



not touched the ectoderm at the lower edge of the stomodseum, and by 

 a mesentery in the lowest part of its length, x600 (p. 808). 



5. Section of the edge of the sidcus, X 600 (p. 808). 



6. Outline of the sulcus in transverse section below the level of the rest of 



the stomodseum ; the azygos tentacle (5) and the directive mesenteries 

 (3) are indicated also (p. 808). 

 In Figs. 2, 3, and 5, the arrow indicates the supposed junction of ectoderm and 

 endoderm. 



• Op. cif. supra. 



Pboc. Zool. Soc— 1897, No. LIV. 54 



