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Show 1897.] PLANKTON OF THE FAEROE CHANNEL. 805 it seemed to me worth while to study all the stages in my power, and to endeavour to put the matter straight again. ^ The following stages have been drawn from ' Besearch specimens and cut into microscopic sections :- gtage. Tentacles. Mesenteries. Oral tentacles. A 6 10 0 B 7 H 4 Appearance of the unpaired tentacle (5). C 9 12 4 D 9 12 4 E 10 14 6 F 11 16 8 G- 12 16 8 H 13 18 8 I ? 19 10 (First appearance of generative cells.) A few older stages have also been studied. This table, taken together with the diagram (Pl. XLVII. fig. 1), sufficiently shows the successive development of the various structures, and their position in the oldest specimens. As regards this diagram, the order of succession of the first four pairs of mesenteries is taken from van Beneden's account of an allied species, and that of the first two pairs of tentacles is inferred from his drawings and descriptions. The facts implied by tbe remainder of the diagram I have myself checked, and they will be found to differ entirely from those given by Vanhoffen, and to agree with those of Boveri ou all points with which we both deal. The developmental order ot the first four pairs of mesenteries, as described by van Beneden (c, a, b, d), appeared at first to contradict the lettering attached to the same mesenteries by Boveri (d, a, b, c), but the latter author courteously informs me that he did not intend by these letters to indicate a develoDinental succession : van Beneden's observed order may therefore be taken to hold good for this species also, m default of direct evidence. ARACHNACTIS BOURNEI, sp. n. There can be no doubt that the specimens from the English Channel, first recorded by Bourne, and described by van Beneden, under the name of Arachnactis albida, belong to another and an unnamed species. Not only are the form and proportions of the animal quite different from those of albida, both in van Beneden's drawings and in a few specimens which I received from the Marine Biological Station at Plymouth in 1893, but also the rate at which different sets of organs are developed is not the same in the two species. This is at once apparent on a comparison of my table of albida stages (given above) with the following :- Tentacles. Mesenteries. tentafl e 9> Van PBeRPnOelCdy emZnoO'uOstL h.o Slsdpoeeccs-it1m e8lna9sr7 v(,a1 N 8.o0.3. ) L7 .I .I8 I .09 10 °3 2 |