362 



ARTHUR WILLET. 



characteristic of the dorsal fin is accurately portrayed in the 

 figure of the species given by Peters ; but he erred seriously in 

 asserting th6 absence of a caudal fin, and in describing the 

 anus as occupying a median ventral position. 



Giinther^ perceived this error on the part of Peters suffi- 

 ciently to justify him in releasing the species from the cum- 

 brous genus to which the latter author had assigned it. 



As a matter of fact, the caudal fin and anal opening have 

 the same relations here as in the European species, only the 

 former structure is much thinner and more delicate in B. 

 cultellum, and can be practically annihilated by unfavorable 

 reagents. 



Explanation. — Branchiostoma cultellum from left side. ch. Anterior 

 and posterior extremities of notochord. e. Eye. m. Mouth, at. Atrio- 

 pore. an. Anus. /. m. Left metapleur. 



Continuity of right metapleur with mesial ridge of ventral fiu is indi- 

 cated, as is also the lateral position of anus and the well-marked 

 caudal fin. 



In many of Professor Haddon's specimens, particularly in 

 the sublimate and osmic acid preparations, the caudal fin, 

 which is perfectly hyaline and of extreme tenuity, is very 

 finely preserved, and in all cases the anus lies on the left side. 



The number of myotomes in B. cultellum is about fifty- 



1 Giiuther (3, p. 33), criticising the account given by Peters, says, "The 

 position of the vent is, at least in some of our specimens, rather lateral than 

 median. Whether these differences are owing to the better state of preserva- 

 tion of our specimens, or related to the difference of locality, I am not pre- 

 pared to decide." Professor Haddon's collection decides this point in favour 

 of the former explanation of the differences in question. 



