Frogs. 39 



and are replaced by an internal series, or gills of the 

 type of those developed in fishes. This modification 

 in the structure of these organs is of considerable inte- 

 rest from a morphological point of view, when we re- 

 member that in sharks there are originally in the 

 embryo distinct external gills, which are lost as the 

 shark attains his more perfect organisation, and are 

 replaced by the permanent gills, which are formed 

 directly on the aortic arches. From these conditions 

 it seems as if external gills were a more primitive or 

 embryonic form, and internal gills a more specialised 

 modification of respiratory organs. The amphibians 

 which show these internal gills are the giant Sieboldia 

 of Japan, which reaches a length of four feet, and the 

 amphiuma and menopoma of North America. The 

 caducibranchiate tailed amphibians are the sala- 

 manders and newts, the latter of which are common 

 in our ditches, where their metamorphoses ^an easily 

 be traced. The common newt is interesting on ac- 

 count of the bright colours which it exhibits at certain 

 stages, and for the remarkable dorsal crest which it 

 also occasionally possesses. 



22. Order 3, Annra. The largest group of the 

 Amphibia consist of the frogs and toads, or the tailless 

 forms. In these, the larva or tadpole loses during 

 its development all traces not only of its gills, but also 

 of its tail ; the hinder limbs are also in these more 

 perfectly developed than the fore, and the two proxi- 

 mal bones of the ankle are elongated, so as to make 

 what appears to be an independent third portion of 

 the hind limb. The fore arm and the leg proper also 

 differ from those of urodeles and of the higher verte- 



