804 



DR. G. HERBERT FOWLER ON THE 



[June 15, 



Arachnactis dlbida, M. Sars. 



M. Sars, Fauna littor. Off Floroe Island. 



Norveg. i. p. 28. 

 Forbes & Goodsir, Tr. Boy. The Minch. 



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Browne (unpublished).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 



56 35' N., 20 19' 



W. 

 Rockall to Hebrides 



(' Holsatia '). 

 Faeroe Channel 



(' Triton '). 

 60 N., 7 W. 



Autumn & winter, 



1846. 

 Aug. 1850. 



Sept. 1861. 

 July 1885. 

 Aug. 1882. 

 Sept. 1893. 



?Rockall to Heb- 1889. 



rides (' National'). 

 North Sea, near 



Brit, coasts. 

 Valentia Island. 

 Faeroe Channel 



('Research'). 



Feb. to April, 1895. 



Mar. 1895. 

 Aug. 1896. 

 July 1897. 



Original description 



of the species. 

 Eefer to Dr. Balfour 



having taken it in 



1841. 

 In strong current 



from N.B. 



German Plankton 

 Exped. 



? A. bournei. 



All recent observers of Arachnactis are agreed that it is to be 

 referred to the Cerianthidce. As regards the early development of 

 this group, Kowalewsky * 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 Russian. 

 Van Beneden 2 , beginning where Kowalevsky left off (and giving 

 a short abstract of his work), traced the development of an Arachn- 

 actis (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 we have also two papers : 

 Boveri 3 began with 8 tentacles and five pairs of mesenteries, and 

 carried it to a stage with 21 tentacles and 14 oral tentacles; 

 Vanhoffen 4 made sections of, and described in detail, a stage with 

 19 mesenteries. 



As regards these two last papers, I am glad to say that my 

 observations bear out those of Boveri, but regret that they are far 

 from agreement with those of Vanhoffen. The latter author has 

 been drawn into a series of mistakes by an initial error, which is 

 best given in his own words : " Die Keihenfolge in der Bildung 

 der Septen ergiebt sich aus der Verfolgung der Schnitte von uuten 

 nach oben"; that is to say, he imagines that the order of develop- 

 ment 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. As 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. 4 Op. tit, supra, 



