250 MR GOODSIR ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 



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 

 the abdominal folds terminate, is continued 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 anus is, as has been observed by MULLER, very remarkable ; but it is in ex- 

 act accordance with a fact mentioned to me by Professor AGASSIZ, that in certain 

 fresh-water fishes, the development of which he had watched, a fin of this kind, 

 with rays, exists for a short period of their embryonic existence, and then dis- 

 appears. 



ANATOMICAL DESCRIPTION OF THE AMPHIOXUS. 



Osseous System. 



Neuro-skekton. The osseous system, properly so called, consists of a " chorda 

 dorsalis" tapering at both ends, without the vestige of a cranium, and of a dorsal 

 and ventral series of cells, the germs of superior and inferior interspinous bones 

 and nn rays. The " chorda dorsalis" consists of sixty to seventy vertebrae, the 

 divisions between which are indicated by slight bulgings, and lines passing obliquely 

 from above downwards on the sides of the column. In this way a separation into 

 individual vertebrse is rather indicated than proved to exist ; for although the 

 column has certainly a tendency to divide at the points above mentioned, yet that 

 division is rather artificial than natural. There is no difficulty in ascertaining 

 above sixty divisions, those at each end above the number stated run so much 

 into one another that no correct result can be obtained. 



The chorda dorsalis is formed externally of a fibrous sheath, and internally 

 of an immense number of laminae, each of the size and shape of a section of the 

 column at the place where it is situated. When any portion of the column is 

 removed, these plates may be pushed out from the tubular sheath, like a pile of 

 coins. They have no great adhesion to one another, are of the consistence of 

 parchment, and appear like flattened bladders, as if formed of two tough fibrous 

 membranes pressed together. 



As the fibres of the sheath are principally circular, provision is made for 

 longitudinal strains on the column by the addition of a superior and inferior ver- 

 tebral ligament, as strong cords stretching along its dorsal and ventral aspects. 

 The superior ligament lies immediately under the spinal cord, and may be recog- 

 nized as a very tough filament, when the column is torn asunder, or some of the 

 vertebrse removed. The inferior ligament may be raised from the inferior surface 

 of the column in the form of a tough ribbon. From the sides of the column apo- 



