368 
B. BAY LANKESTER. 
and Schneider (6). The true relations are shown in the 
drawing, PI. XXXVI A, fig. 2. 
Number of Myotomes. — The number of myotoines is an 
important and fundamental numerical character of the species 
of Amphioxus. In the Amphioxus lanceolatus of Naples 
there are sixty-one of these myotomes. The last myotome is 
extremely delicate, and it is by no means easy to count the 
whole series with certainty. In some large specimens I have 
counted sixty-two myotomes. 1 Dr. Gunther (7) in his im- 
portant account of the genus Branchiostoma (= Amphioxus) 
in the ‘ Report on the Zoological Collections made in the Indo- 
Pacific Ocean during the voyage of H.M.S. “ Alert/’ ’ published 
by the trustees of the British Museum, 1884, gives sixty as 
the number of myotomes in specimens of A. lanceolatus 
of Naples, fifty-nine in one from Polperro, and sixty-one in a 
specimen from the Scandinavian coast. The number in other 
species varies as follows : — In A. elongatus from the coast 
of Peru, 79; in A. bass an us from Bass’s Straits, 75 or 76; in A. 
Belch eri from the coast of Borneo, 64 or 65 ; in A. caribaeus 
from Rio de Janeiro, 60 or 59; in A. cultellus of Peters, 52. 
It appears that the full number of myotomes is acquired by 
Amphioxus at a very early period in its growth, even before 
(?) the complete formation of the epipleural chamber. It is 
not quite certain that the number of myotomes varies in A. 
lanceolatus from fifty- nine to sixty-two, as would appear 
from the numbers above given. The discrepancy may be due 
to the difficulty of accurate counting, and to the recognition or 
omission of the terminal myotome. The question, therefore, 
needs some further study. 
Position of Mouth, Atriopore and Anus. — The mouth of 
Amphioxus is that small median aperture surrounded by a 
well-developed sphincter muscle which is concealed by the oral 
hood. It is not correct to call the margin of the wide space 
1 I Lave a note of the number of myotomes counted in four large specimens 
besides those referred to in the text, viz. respectively fifty-nine, sixty-two, 
sixty-one, and sixty-one. In the figure in Plate XXXIV I have given sixty- 
two myotomes, but sixty-one is the usual number. 
