On Arachnactis brachiolata. 525 



Art. XL — On Arachnactis brachiolata^ a Species of Float- 

 ing Actinia found at Nahant, Massachusetts. By A. 



Agassiz. 



[Communicated Oct. 1, 18G2.] 



Like the Norwegian species mentioned by Sars,* our 

 Arachnactis is found in the autumn ; the first specimens 

 occurred about the beginning of September, and had only 

 four tentacles. (Fig. 1.) They are exceedingly abundant 

 at night; large numbers were caught fii;. i. 



every evening with the dip net.f Their 

 general appearance is so like a Bra- 

 chiolaria, that, at first not recognizing 

 it as a Polyp, I supposed it to be the 

 larva of one of our star-fishes. The 

 manner in which they move is precise- Young Amriumrtis, narrow 

 ly similar to that of the larva of one ol'"^"^'"''''^ P'''*' "^ tentacles. 

 our common star-fishes, the Aster acanthion herylinus Ag. 

 They always swim with the disk turned down, the abacti- 

 nal region pointing up, and the longitudinal axis of the 

 body inclined. Their movements are very slow, as up to 

 the time when they have sixteen tentacles, and have 

 reached a length of about one eighth of an inch, they pro- 

 pel themselves almost entirely by means of the numerous 

 ciliae which are scattered on the surface of the tentacles. 



The tentacles themselves are not capable of extensive 

 contractions ; they can simply be slightly shortened, and 

 curved over the opening of the mouth, and are never 

 drawn into the disk as in Actinia. The column itself con- 

 tracts only little, even when irritated by touch. The body 

 is perfectly transparent, so that the partitions can readily 

 be seen through the outer wall. They extend from the 



* Sars, M. Fauna littoralis Norvegi^, Erste Lieferung. Christiana, 1846. 

 t Agassiz, A. Proc. Bost. Soc. Nat. Hist. Vol. ix. p. 159. 



JOURNAL B. S. N. H. FEBRUARY, 1863. 



