OCR Text |
Show 1897.] PLANKTON OP 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 m 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 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 Beueden 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 (' Holsatia' 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 tbe 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 leDgth, x600 (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 4, the arrow indicates the supposed junction of ectoderm and endoderm. Op. cit. supra, j |