[Proc. Roy. Soc. Victoria, 22 (N.S.), Pt. I., 1909.] 



Art. Vin. — Xotes on the Structure of Asymmetron 

 hassanum, Gilnther. 



By ETHEL REMFREY MORRIS, M.Sc, 



AND 



JANET RAFF, B.Sc. 



(Plates XVIII.— XX.). 



[Kead 8th July, 1909.] 



Whilst working in the Biological Laboratory of the Mel- 

 bourne University, our attention has been drawn to certain 

 points in the structure of Asym7netron hassanum, which either 

 have not hitherto been noticed, or have not been adequately 

 described. This species is not uncommonly met with at 

 depths up to twenty fathoms in certain rather restricted areas 

 along the Victorian coast, and, as it is commonly employed in 

 our laboratories in substitution for the genus Amphioxus, the 

 description of which is given in all text-books, we have, at 

 the suggestion of Professor Spencer, placed on record the fol- 

 lowing brief notes: — 



1. — External Structure. 



The average length of twelve specimens is 39 mm. ; the 

 average number of myotomes, 75 ; the average myotome for- 

 mula, 44, 17, 14. 



The ventral fin has chambers and fin rays, which are single 

 (Fig. 4), but which may appear double when cut at certain 

 angles (Fig. 9). The rostral fin is large. 



At the anterior end there is always present an incomplete 

 ring of pigment (Fig. 11), just in front of the first myotome. 



The oral cirri are 24-26 in number, and bear sense papillae. 

 The velum is placed at the level of the seventh myotome, and 

 bears 16-18 tentacles. 



