STEUCTUKAL DISTINCTIONS. 9 



of the heart, really or apparently, by a single trunk. In Lepi- 

 dosiren the veins from the lung-like air-bladders traverse the 

 auricle which opens directly into the ventricle. In some the vein 

 dilates before communicating with the ventricle into a small 

 auricle, which is not outwardly distinct from the much larger 

 auricle receiving the veins of the body. In Proteus the auricular 

 system is incomplete. In Amphiuma the auricle is smaller and 

 less fringed than in the Sirens, the ventricle being connected to 

 the pericardium by the apex as well as the artery. This forms a 

 half spiral turn at its origin, and dilates into a broader and 

 shorter bulb than in the Sirens. 



" The pulmonic auricle," continues the learned Professor, "thus 

 augments in size with the more exclusive share taken by the lungs 

 in respiration ; but the auricular part of the heart shows hardly 

 any outward sign of its diversion in the Batrachians. It is small 

 and smooth, and situated on the left, and in advance of the ventricle 

 in Newts and Salamanders. In Frogs and Toads the auricle is 

 applied to the base of the ventricle, and to the back and side of 

 the aorta and its bulb." 



In the lower members of the order, the single artery from the 

 ventricle sends, as in fishes, the whole of the blood primarily to the 

 branchial organs, during life, and in all Batrachians at the earlier 

 aquatic periods of existence. In the Newt three pairs of external 

 gills are developed at first as simple filaments, each with its 

 capillary loop, but speedily expanding, lengthening, and branching 

 into lateral processes, with corresponding looplets ; those blood- 

 channels intercommunicating by a capillary network. The gill is 

 covered by ciliated scales, which change into non- cilia ted 

 cuticle shortly before the gills are absorbed. In the Proteus 

 anguinus, three parts only of branchial and vascular arches are 

 developed, corresponding with the number of external gills. In 

 Siren lacertina the gills are in three pairs of branchial arches, 

 the first and fourth fixed, the second and third free, increasing in 

 size according to their condition. 



The AMPHIBIA, then, have all, at some stage of their existence, 

 both gills and lungs co- existent : respiring by means of branchiae 

 or gills while in the water, and by lungs on emerging into the 

 open air. 



