ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 375 



Having now stated what has already been done in the 

 anatomy of this remarkable animal, I shall proceed to de- 

 scribe in detail the structure of the specimen I have depicted, 

 reserving for the concluding part any general observations 

 I may have to make on its structure and zoological cha- 

 racter. 



The dimensions and weight of the specimen of Amphioxus 

 which has afforded the materials for this paper, are, length two 

 inches ; depth, a little before the middle, two lines ; weight eight 

 grains. The other specimen in my possession is half-an- 

 inch shorter, and not so favourable for examination. They 

 were dredged up by Mr. Forbes from a sandbank, in deep 

 water, on the east coast of the Isle of Man, were extremely 

 active, and resembled, on superficial inspection, small sand- 

 eels. On each side of the abdomen are two longitudinal 

 membranous folds, and behind them an anal fin, omitted in 

 Mr. Tarrell's description. The folds commence, minute, on 

 each side of the hyoid apparatus, pass back on the sides of 

 the abdomen, increasing in breadth till they are as broad as 

 one-fifth of the depth of the animal ; they then diminish and 

 terminate at the point where the lateral muscles approach on 

 each side of the intestine, that is, at the junction of the 

 middle and posterior thirds of the animal. 



The anal fin is a fold of integument, which, commencing 

 at the point where tin; abdominal folds terminate, is con- 

 tin in '(I to the anus, where it is interrupted, but reappearing 

 behind it, and becoming broader, passes on to be continuous 

 with the dorsal fin at the extremity of the tail. The existence 

 of a median fin in front of the amis is, as has been observed 

 by Miiller, very remarkable ; but it is in exact accordance 

 with a fact mentioned to me by Professor Agassiz, that in 

 certain fresh-water fishes, the development of which he had 

 watched, a tin of this kind, with rays, exists for a short period 

 cf their embryonic >■'■ istence, and t hen disappeai 



