xiii PHYLUM CHORDATA 283 



pair of horny jaws. The enteric canal grows to a great length 

 and is coiled like a watch-spring, and the tadpole browses upon 

 the water-weeds which form its staple food. 



Soon the external gills show signs of shrivelling, and at the 

 same time internal gills, like those of Fishes, are developed in the 

 branchial clefts. A fold of skin, the operculum, appears on each 

 side, in front of the gills, growing from the region of the hyoid 

 arch, and extending backwards until the gill-slits and external gills 

 are covered and there is only a single small external branchial 

 aperture on each side, as in Holocephali (3, 4). On the right side 

 the operculum soon unites with the body-wall so as to close the 

 branchial aperture, but on the left side the opening remains for 

 a considerable time as the sole means of exit of the water. At 

 this time the tadpole is to all intents and purposes a Fish. 



The lungs now appear, and the larva is for a time truly 

 amphibious, rising periodically to the surface to breathe air : the 

 single branchial aperture, however, soon closes, and henceforth 

 respiration is purely aerial. 



In the meantime the limbs are developed. The hind-limbs 

 appear as little rounded buds, one on each side of the root of the 

 tail (5). The fore-limbs arise beneath the operculum and are 

 therefore hidden at first ; soon, however, they emerge by forcing 

 their way through the operculum. As the limbs increase in size 

 the tail undergoes a progressive shrinking (6-8). The mouth 

 widens by the backward rotation of the suspensorium, the intestine 

 undergoes a relative diminution in length, and vegetable is exchanged 

 for animal diet. The little, tailed Frog can now leave the water 

 and hop about upon land ; its tail is soon completely absorbed, and 

 the metamorphosis is complete. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Amphibia are Craniata which, in nearly all cases, possess 

 gills either in the larval state only or throughout life, and which 

 usually breathe by lungs in the adult condition. The skin is 

 glandular, and there may or may not be a bony dermal exoskeleton. 

 When unpaired fins are present, they are never supported by 

 fin-rays. The paired appendages, when present, are pentadactyle 

 limbs : the digits are usually devoid of claws. The skull is auto- 

 stylic and is articulated with the first vertebra by paired occipital 

 condyles borne on the exoccipitals. The basi-occipital and supra- 

 occipital are usually, and the basisphenoid is always, absent : 

 there is a large parasphenoid, and there are well-developed 

 paraquadrates (squamosals). In the branchiate forms large hyoid 

 and branchial arches- persist throughout life : in the non-branchiate 

 species these structures undergo more or less degeneration and 

 give rise to the hyoid-cartilage. The heart has a sinus ven.osus,, 



