ALIMENTARY CANAL IN LEPIDOSIEEN AND PEOTOPTEEUS. 511 
would thus become medio-dorsal except the pneumatic duct 
or trachea, which would lead round the right side of the 
oesophagus or pharynx to the still ventral glottis. 
(2) Correlated with the increased size of the right lung in 
Polypterus we find that the left vagus takes a share in its 
innervation, a stout, in fact the main, branch of this nerve 
passing across to the right lung dorsal to the oesophagus. 
Bearing in mind the mode of development of the lung from 
a ventrally placed rudiment it need hardly be pointed out 
that this dorsally placed connection between left vagus and 
right lung must be a secondary development. How it has 
come about does not concern the argument, but it may well 
have been owing to the short circuiting of nerve impulses 
through the nerve plexus of the pharyngeal wall having 
caused that part of the plexus which formed the path of the 
impulses to become enlarged so as to form a distinct nerve 
trunk. 
(3) The glottis or opening from pharynx is no longer 
symmetrical in the adult Polypterus in relation to the two 
lungs. It has undergone a shifting towards the right side, 
and is in line with the larger right lung. 
The two first of the three points just established have an 
important bearing upon the comprehension of the conditions 
seen in the Lung-fishes. (1) shows us how the condition of 
the Lung-fishes with their dorsal lung communicating with a 
ventral glottis round the right side of the alimentary canal is 
just a natural step beyond the condition actually existing in 
Polypterus. (2) does away entirely with the at first sight 
apparently insuperable objection to adopting this as a phylo- 
genetic hypothesis involved in the peculiar nerve supply of 
the lung in the Lung-fishes — where (text-fig. 11 , b) the pul- 
monary branch of the left vagus extends on to the actually 
right lung by a path which is dorsal to the oesophagus — for 
we see that in Polypterus (in which the lung retains the 
assumed ancestral condition) there already exists a similar 
prolongation of the left vagus dorsal to the oesophagus to the 
