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Show 804 DR. G. HERBERT FOWLER ON THE [June 15, Arachnactis albida, M . Sars. M. Sars, Fauna littor. Norveg. i. p. 28. Forbes & Goodsir, Tr. Roy. Soc. Edin. xx. 307. Vogt, Archives de Biologie, viii. p. 1. Boveri, Zeit. wiss. Zool. xlix. p. 459. Vanhoffen, Bibliotheca Zoologica, Hft. xx. Browne (unpublished). Fowler Off Floroe Island. Autumn & winter, 1846. The Minch. Aug. 1850. 56° 35' N., 20° 19' W. Rockall to Hebrides (' Holsatia'). Faeroe Channel (' Triton'). 60° N., 7° W. ? Rockall to Hebrides ('National'). North Sea, near Brit, coasts. Valentia Island. I Faeroe Channel -I (' Research'). Sept. 1861. July 1885. Aug. 1882. Sept. 1893. 1889. Feb. to April, 1895. Mar. 1895. Aug. 1896. July 1897. Original description of the species. Refer to Dr. Balfour having taken it in 1841. In strong current from N.E. German Plankton Ex ped. ? A. bournei. All recent observers of Arachnactis are agreed that it is to be referred to the Cerlanthldce. A s regards the early development of this group, Kowalewsky l traced it from the gastrulation to the formation of two pairs of tentacles and one pair of mesenteries ; but unfortunately obscured his information by writing in Bussian. Van Beneden2, beginning where Kowalevsky left off (and giving a short abstract of his work), traced the development of an Arachnactis (apparently not A. albida, although described under that name) from a stage with two pairs of tentacles and one of mesenteries up to a stage with seven tentacles and four pairs of mesenteries. On the later development w e have also two papers : Boveri3 began with 8 tentacles and five pairs of mesenteries, and carried it to a stage with 21 tentacles aud 14 oral tentacles; Vanhoffen 4 made sections of, and described in detail, a stage with 19 mesenteries. As regards these two last papers, I a m glad to say that my observations bear out those of Boveri, but regret that they are far from agreement with those of Vanhoffen. Tbe latter author has been drawn into a series of mistakes by an initial error, which is best given in his own words :-" Die Beihenfolge in der Bildung der Septen ergiebt sich aus der Verfolgung der Schnitte von unten nach oben "; that is to say, he imagines that the order of development of the mesenteries can be inferred from a comparison of their absolute length at a late stage: and it is hardly necessary to say, not only that this assumption is quite unjustifiable, but that the order of development which he consequently assigns to the mesenteries proves to be absolutely erroneous when tested by successive stages. A s Vanhoffen is the latest writer on Arachnactis, 1 Nachrichten d. Ges. d. Freunde d. Naturerkenntniss u. s. w., Moscow, 1873. 2 Archives de Biologie, xi. p. 115. 3 Op. cit. supra. * Op. cit. supra. |