REPORT ON A COLLECTION OF AMPHIOXUS. 363 



two, but it appears to vary within narrow limits, namely, from 

 fifty-one to fifty-five. Giinther formulates them with reference 

 to the position of atriopore and anus as follows : 

 32 + 10 + 10 = 52, 

 or 31 + 11 + 10 = 52. 



As the position of these apertures stands in no causal 

 relation to any particular myotomes, it is difficult and perhaps 

 impossible to give an unvarying statement with reference to 

 this point. 



In one or two instances I have counted as follows : — 



32 + 12 + 8 = 52, the anus being placed at the base of the 

 forty-fourth myotome (cf. accompanying figure). In another 

 case, in which I counted fifty-five myotomes, the formula was 



33 -f- 11 + 11 = 55. Still, in several other instances I have 

 counted as many as fifty-four myotomes. 



The average length of the specimens in Professor Haddon's 

 collection may be placed at 2*5 cm., although several indi- 

 viduals measured upwards of 3 cm., two of them attaining a 

 length of nearly 3'5 cm. Peters stated the length of his 

 specimens, which came from Moreton Bay, to be 2*3 cm. 

 Beyond a few measurements Peters gives no numerical data 

 whatever, and no account of the internal organs. 



There are from twenty-four to twenty-seven ventral fin- 

 chambers between atriopore and anus. External examination 

 from the ventral aspect gives rise to the impression that the 

 ventral fin in this species contains paired fin-rays, such as are 

 known to occur in B. lanceolatum. This optical eff'ect is 

 due to the fact that the caudal fin is continued forwards as a 

 mesial ridge below the ventral fin-chambers, and so produces a 

 double appearance in the latter in a surface view of the ventral 

 aspect. As in B. lanceolatum, the fin-chambers are always 

 single median spaces; but while in the former each of them 

 contains a pair of gelatinous fin-rays suspended from its dorsal 

 wall, in B.cultellum the ventral fin-chambers are desti- 

 tute of fin-rays. 



The absence of ventral fin-rays has recently been also 

 observed by E. A. Andrews in a new species of Amphioxus 



