552 ORGANS OF RESPIRATION 1 . 



tensions, with a middle stem from which the soft ramified 

 compressed filaments or laminae hang free on each side. 



By the movements of the lower jaw and os hyoides, the 

 respiratory currents of water are directed inwards by the 

 mouth, and outwards by the lateral branchial openings, as in 

 fishes ; and the stratum of water in contact with the surface 

 of the external pendent branchiae, is further renewed by the 

 constant vibration of the minute cilia closely covering their 

 whole exterior, as in the branchiae of inferior classes. Al- 

 though the branchial laminse are not here fixed at their outer 

 ends as in cyclostome and in plagiostome fishes, the number 

 of exterior lateral branchial openings varies in different 

 species, there being but one on each side of the neck in the 

 larvae of anurous amphibia, two in the proteus, three in the 

 siren, and four in the triton. The branchiated period of the 

 larvae of salamandra is commenced within the oviducts of the 

 female, and that of pipa is passed within the cells of the 

 parent's back ; in most others it is passed in the water, where 

 the ciliary action of the entire naked cutaneous surface of the 

 body, compensates for the imperfect development of other res- 

 piratory organs. Even in the adult state, frogs have lived 

 thirty hours after the entire removal of their lungs, breathing 

 then solely by their naked cutaneous surface, which is also 

 their sole respiratory organ during months of hybernation, 

 immersed in mud under water during winter. 



In the proteus, the axolotus, and other amphibia which 

 permanently retain the gills, there are two long membranous 

 respiratory air-sacs, rudimentary lungs, extending, like the 

 air-sac of fishes, far backwards into the cavity of the abdo- 

 men, above the other viscera, but freely moveable in the 

 cavity of the peritoneum, and covered with that serous mem- 

 brane. The small pulmonary arteries descend from the 

 posterior branchial trunks of the aorta, their ramifications 

 form a close network over the entire surface of the pulmo- 

 nary sacs, and the aerated blood is returned by the pulmo- 

 nary veins to the small left auricle of the heart. The lungs 

 are here simple sacs, without internal cells, and they commu- 

 nicate with the pharynx by thin membranous smooth ductus 

 pneumatici, or tracheae, still nearly destitute of all cartilaginous 

 rings or annulated appearance. This simple condition of the 

 lungs and trachea is seen also in the triton in the adult state, 

 but is only a transient condition of these organs in the larvae 



