THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 69 



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 

 verm, 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 branchiae of the scorpion group are directly derived 



V.br. 



-br.cart^/ \ 



br.cart. 



Bro. 



Fig. 68. — Diagram of three Branchial Segments of Ammoccetes (A) compared 

 with three Branchial Segments after Transformation (B) to show how 

 the Branchial Appendages of Ammoccstes form the Branchial Pouches 

 of Petromyzon. (After Nestler.) 



In both figures the branchial cartilages (br. cart.), the branchial view (V. br.), and the 

 sense-organs (S), 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 Ammocoates 

 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 



