42 AMPHIBIA CHAP. 



half of the three branchial arches, just near the upper corners of 

 the clefts ; and the skin of the body is continued upon the stem 

 of each gill, pigmented like the rest of the surface of the body. 

 Such a gill is more or less like a blade, standing vertically, and 

 is composed of a stem of connective tissue, thick at the base, and, 

 as a rule, carrying two series of fine lamellae, which, however, do 

 not form two opposite series, but hang downwards, being, so to 

 speak, folded down, so that the upper surface of the stem is bare, 

 and carries the lamellae on its under side. In the Axolotl some 

 of these lamellae are further subdivided. In Necturus they are 

 enormously increased in numbers, but are rather short, and they 

 stand no longer in two rows, but are crowded into one. Those 

 of Proteus form two rows of dendritic filaments ; those of Siren 

 are likewise much ramified. 



The larvae of the Urodela have four clefts. In the adult 

 Siren these are reduced to three, the first, namely, that between 

 the hyoid and the first branchial arch, being closed up. In 

 Necturus, Proteus, and Typlilomolge the clefts are further reduced 

 to two, owing to the closing up of the first and last, only those 

 between the first, second, and third arches remaining. Amphiuma, 

 and usually Cryptobranchus alleghaniensis, possess only one pair 

 of clefts, while in C. japonicus and in the Salamandridae all the 

 clefts are abolished. 



The gills of the Urodela are always uncovered, although a 

 short operculum is formed from the posterior margin of the 

 hyoidean arch ; the halves of this fold meet below the throat, 

 and persist in various terrestrial and aquatic species as the " gular 

 fold." It reaches its greatest size just before metamorphosis, but 

 scarcely ever produces a proper outer gill-chamber, and it does 

 not cover the gills owing to their rather pronounced dorsal 

 position. It is perhaps best developed in Typlilomolge, and even 

 there its dorsal portion is continued upon the first of the three 

 broad vertical and short-fringed blades which form the gills. 



A description of the gills of the Apoda will be found in the 

 systematic part. 



In the Anura the gills are complicated, owing to the develop- 

 ment of the so-called internal gills. First appear, exactly in the 

 same way as in the Urodela, the external gills, one upon each of 

 the first three branchial arches. In the larva of Rana esculenta, 

 5 mm. in length, a little protuberance appears upon the first, 



