550 ORGANS OF RESPIRATION. 



aquatic insects in the larva state ; it is still membranous, 

 like that of a proteus, or of a lepidosiren ; generally it is 

 single, as in most higher vertebrata, but is sometimes double, 

 as it is in many chelonia ; and in some fishes it opens as 

 high as the pharynx like the trachea of other vertebrated 

 classes. 



Thus, while the conditions of the branchial and pulmonary 

 organs of fishes present a perfect adaptation for the dense ele- 

 ment they are destined permanently to inhabit, they have many 

 analogies with the forms of the gills and air-sacs of mollusca, 

 and other invertebrated tribes, and also the closest affinities 

 with the deciduous and permanent respiratory organs of the 

 amphibia next above them in the scale. In the successive 

 development, and in the different permanent forms of the 

 branchial subdivisions of the great aortal trunk issuing from 

 their single ventricle, they exhibit the embryo conditions 

 of this vessel in all the higher vertebrata ; and in the various 

 conditions of their air-sac and ductus pneumaticus, they 

 imitate the embryo forms of the pulmonary organs in all the 

 higher air-breathing classes. 



The amphibious animals breathe at first, like fishes, by 

 means of branchiae ; their air-sac is double, and is now more 

 obviously subservient to aerial respiration, and they respire 

 also by the entire surface of their naked, sensitive, and 

 vascular skin. In the earliest condition of the tadpole, when 

 the whole body is covered with vibratile cilia, small ciliated 

 branchial filaments are seen extending from the sides of 

 the neck, which produce currents of water over their surface 

 by the action of their minute vibratile organs ; and in this 

 first external filamentous condition of the gills, they may be 

 compared to the earliest deciduous external branchial filaments 

 seen in many cartilaginous and osseous fishes. Around the 

 margin of each of these short simple primary filaments, a single 

 capillary blood-vessel extends ; and by the successive forma- 

 tion of loops on this vessel, small ramifications originate and 

 develope along the sides of each branchial filament each 

 branchial ramification having only a single capillary blood- 

 vessel around its margin. Before the development of the 

 pulmonary organs and the left auricle, the venous blood of 

 the system in the tadpoles of amphibia, is sent through a 

 bilocular heart, a bulbus arteries us, and branchial arteries, 

 as in the class of fishes ; and in many of the larvee of am- 



