GILLS AND GILL CLEFT 



glands of the amphibian. On the other hand, the also 

 burrowing Ccecilians already referred to have scales. 

 But though the Coecilians have scales they have also 

 the mucous glands of other amphibia. It is doubtful, 

 however, whether some of the heavily armoured am- 

 phibia of remote antiquity, the Labyrinthodonts and 

 their allies, could have possessed much in the way of 

 slime-producing glands. 



As to the nature of the limbs, which certainly dis- 

 tinguish so far as their characters go, the Menobranchus 

 and the Lacerta, there is no possibility whatever of 

 drawing by their help a hard and fast line between 

 amphibia and reptilia generally. In both groups we 

 have reduction of number of digits and of the limbs 

 themselves, culminating in both divisions alike in com- 

 pletely apodous forms. When however we come to 

 the respiratory apparatus, we find at once a difference 

 which holds good throughout the whole reptilian and 

 amphibian series with insignificant exceptions. We 

 may safely state that no reptile ever breathes by means 

 of gills, and that in no reptile is there ever any perman- 

 ence of the gill clefts. On the other hand in amphibia 

 there may be a sharing of the respiratory functions for 

 life between gills and lungs, and at some time of its life 

 the amphibian breathes only by gills. Moreover there 

 is often a persistence of the gill clefts up to mature life, 

 a feature which the amphibian clearly shares with the 

 fish ; in which these clefts, putting the outside world 

 into communication with the pharynx, always persist. 

 In the fish these clefts are divided by bars of cartilage 

 which bear vascular tufts which are the gills. In all the 

 higher vertebrates the embryo shows traces of these 

 clefts ; but in the amphibia only do they ever persist 

 into adult life. This leads us to the consideration of the 

 development of the amphibia as compared with that 

 of the reptilia. In the latter the egg is always large 



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