200 Proceedmr/s of the Royal Physical Society. 



Further, as I have pointed out elsewhere,^ many of the 

 groups without external gills are those which possess a yolk- 

 sac with highly vascular surface, which must itself act as 

 a highly efficient respiratory organ, in addition to its nutritive 

 function. It is quite to be expected in such a case that 

 external gills, with their purely respiratory function, should 

 disappear. 



We see, then, that there is nothing improbable in the 

 disappearance of external gills. On the other hand, it 

 appears to me most 'unlikely that they should have developed 

 independently in the three groups in which they occur. 

 They show a remarkable similarity — not a superficial re- 

 semblance, but a deep-seated resemblance — in their funda- 

 mental characters. They are situated in the same relation to 

 their visceral arches : the aortic arch passes out into them in 

 exactly the same way. 



Enteric Organs of Eespieation. 



In a few cases, the general enteric lining plays an important 

 part in respiration, e.g., in some of the loaches this is the 

 case. The gut lining is very highly vascular, and the living 

 fish is seen at intervals to sivalloiu bubbles of air ; if not 

 allowed to do this, the fish dies. Again, in the Siluroid 

 Callichthys, water is taken in periodically to the posterior 

 part of the intestine, which acts as an organ of respiration. 



It is, however, normally in the region of the pharynx that 

 we find respiratory activity specially concentrated. We find 

 in this region the series of gill-clefts with their respiratory 

 walls, their surface increased by the development of the 

 highly vascular lamellae. We are still in complete obscurity 

 regarding the phylogeny of the gill-clefts. In ontogeny 

 they arise as a series of pockets or pouches of the 

 pharyngeal wall, which secondarily fuse at their tips with 

 the ectoderm, and open to the exterior. Now the great 

 question to be answered regarding their phylogeny, and one 

 to which we cannot as yet find any certain answer, is. Were 

 the gill-clefts in their early functional stages pouches or clefts ? 



1 Proc. Camh. Phil. Soc, vol. x. p. 233. 



