THE EVIDENCE OF THE RESPIRATORY APPARATUS 169 
of the water for respiration, This is manifestly the wrong way to 
look at the matter: the adult form is derived from the larval, not vice 
versd, and the transformation process shows exactly how the gills, 
in Rathke’s sense, come together to form the bronchus and so make 
the gill-pouches of Petromyzon. 
When we bear in mind that almost all observers consider that 
the internal branchie of the scorpion group are directly derived 
V. br. 
Fic. 68.—D1aGRaM OF THREE BRANCHIAL SEGMENTS OF AMMOCaITES (A) COMPARED 
WITH THREE BRANCHIAL SEGMENTS AFTER TRANSFORMATION (B) TO SHOW HOW 
THE BRANCHIAL APPENDAGES OF AMMOC@TES FORM THE BRANCHIAL PoucHEs 
or Pretromyzon. (After NESTLER.) 
In both figures the branchial cartilages (br. cart.), the branchial view (V. br.), and the 
sense-organs (8), are marked out in order to show corresponding points. The 
muscles, blood-spaces, branchial arteries, etc., of each branchial segment are 
not distinguished, being represented a uniform black colour. Bro., the bronchus 
into which each gill-pouch opens. 
from branchial appendages of a kind similar to those of Limulus, it 
is evident that a. branchial appendage such as that of Ammoccetes 
might also have arisen from such an appendage, because in various 
respects it is easier to compare the branchial appendage of Ammo- 
ccetes, than that of the scorpion group, with that of Limulus. 
In the case of the scorpions, various suggestions have been made 
as to the manner in which such a conversion may have taken place. 
The most probable explanation is that given by Macleod, in which 
