AMPHIBIA 231 



disappears. Such is the tadpole larva, and it serves as a fish- 

 like stage to a creature which finally becomes a lung- breathing 

 animal. 



Of the gill clefts the first pair, or spiracles, are not put into 

 communication with the exterior, but this internal pouch of 

 the endoderrn is converted into the Bustachian tube and the 

 middle ear. Four clefts are perforated behind this one. With 

 these clefts are associated the hyoid and four branchial arches. 

 The hyoid arch is developed behind the spiracle, and the 

 branchial arches behind the corresponding branchial clefts. 

 The external gills are produced from the first and second 

 branchial arches, and these are followed later by a smaller 

 outgrowth of the third arch and small protrusions of the fourth 

 arch. These function at the early period of larval life. In 

 about three days after hatching the mouth and the branchial 

 clefts are perforated. The latter develop internal gills in the form 

 of tufts of the walls of the margins of the clefts. Eespiration is 

 now carried on by the water being brought into the pharynx 

 through the mouth and passed over the internal gills. Just 

 before this happens the wall of the hyoid region grows out as 

 a backwardly directed fold on each side to form the operculum, 

 or gill cover, and the water escaping from the gills passes 

 out at a single opening. This is different from the conditions 

 in the skate and its allies, but it is exactly parallel to what 

 happens in the majority of fishes. The extension of the 

 operculum is accompanied by a degeneration of the external 

 gills, and the gill opening of the right side is closed completely 

 during a period when the left gill opening remains patent and 

 functional, discharging both gill chambers. 



The mouth of the tadpole is furnished with horny jaws, 

 upper and lower, supported by the cartilages of the jaws, 

 and at the sides the mouth is produced into horny papillae. 

 The horny apparatus is derived from a cornification of the 

 ectoderm, and it is lost by a shedding of this layer at meta- 

 morphosis. As the tadpole increases in size and gains in 

 strength the adhesive organ degenerates. It becomes split 

 into two, decreases, and finally disappears. Cutaneous sense 

 organs are developed in rows on the larva, forming a lateralis 

 system which is lost at metamorphosis. 



