150 THE WAYS OF BREATHING 



the latter are another generalised form of re- 

 spiratory organ which may have arisen de novo 

 in different divisions of the animal kingdom, or 

 whether gill-clefts are not rather a specialised 

 formation, homologous throughout, like the 

 Molluscan ctenidium. The analogies of the 

 cutaneous branchiae and tracheae render the 

 question a legitimate one for discussion and 

 an extremely difficult one to settle. Many 

 zoologists, of whom I happen to be one, think 

 that the gill -cleft is a monophyletic structure, 

 and in order to give effect to this point of 

 view I suggested, as a memoria technica, the 

 phylogenetic term Branchiotrema, 1 to include all 

 animals possessing gill-clefts at any period of 

 their life-history. 



The extraordinary persistence of gill - clefts 

 and gill-pouches in the embryos of the higher 

 lung-breathing vertebrates, the constancy of their 

 innervation in all Craniate vertebrates, and the 

 combination of structures which accompany them 

 in all Chordates, speak for their homogeneity. 

 Whilst we may admit that the general homology 

 of gill-clefts is open to question, we must at the 

 same time assert that the burden of proof to the 



1 Cf. A. Willey, " Enteropneusta from the South Pacific," Zoo- 

 logical Results, part iii., Cambridge, 1899, pp. 223-334. See also 

 R. C. Punnett's memoir on the Enteropneusta from the Maldive 

 and Laccadive Islands in Stanley Gardiner's " Fauna and Flora 

 of the Maldive and Laccadive Islands," 1904-1905. 



