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Show 1897.] PLANKTON OF THE FAEROE CHANNEL. 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. "I Numbered in order of Digestive: 2, 5, 7, 9, 11, &c. / successive development. The differentiation of the filaments of the two kinds of mesenteries in the adult Cerlanthus is apparently not mentioned by the brothers Hertwig1; their figure 3, pl. viii., practically unites the main features of m y figures 2 and 3. Unfortunately, the specimens of Cerlanthus at m y disposal are not very well preserved, but even in them it is obvious that there is a differentiation of the two filaments, of the same kind as, although not precisely identical with, that which I have described above for Arachnactis. Very young germ-cells are recognizable in both types of mesentery in the adult. I have seen nothing in Arachnactis of the small " directive" mesenteries, not attached to the stomodaeum, which are mentioned by von Heider as occurring in Cerlanthus. NOTE.-Since the MS. left my hands, I have received a letter from m y friend Prof. Karl Brandt of Kiel, which informs m e that Prof, van Beneden 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 m e that the genus appears to have been widely taken in the North Atlantic (' National') and in the North Sea (' Holsafia ' 1885, Nordsee Expedition 1895). EXPLANATION OF 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 has not touched the ectoderm at the lower edge of the stomodaeum, and by a mesentery in the lowest part of its length, X 600 (p. 808). 5. Section of the edge of the sulcus, X 600 (p. 808). 6. Outline of the sulcus in transverse section below the level of the rest of the stomodaeum ; 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. 1 Op. cit. supra. PROC ZOOL. Soc-1897, No. LIV. 54 |