The Zoologist — Decemher, 1869. 1935 



siibslitution of one organ for another, though both discharge their 

 functions in the same manner, inasmuch as the respiration is just as 

 aquatic in character after the alteration as it was before it. But the 

 n)odifications of the respiratory apparatus do not cease here : before 

 the tadpole can become a frog, it must do away with these second 

 gills and replace them by lungs; and at the necessary time, a set of 

 changes takes place analogous to those we have already described. 

 The vascular tufts are atrophied, and the lungs, which till now were 

 solid and rudimentary, open up and increase in size. The circulatory 

 organs are correspondingly modified. The calibre of the large 

 branchial vessels is diminished, and the pulmonary trunks increase in 

 number and diameter. Later on, the solid parts of the branchial ap- 

 paratus disappear also, the bones and cartilages being gradually re- 

 absorbed. Eventually the alteration is fully accomplished, and there 

 remains not the slightest trace of the former branchial apparatus. In 

 this instance, not only has there been transformation and substitution, 

 but an actual metamorphosis has occurred ; for the respiration, which 

 was aquatic before, has become atmospheric, and, strictly speaking, 

 the animal, from having been a fish has been converted into a batra- 

 chian. If we examine any particular apparatus, we shall find it also 

 presenting many curious phenomena in the course of its develop- 

 ment. We shall find that as the herbivorous habits give place to 

 carnivorous ones, the digestive apparatus undergoes a change adapt- 

 ing it to the new form of diet: the moulh increases in size and gape ; 

 the little beak organs, or, more correctly, the horny lips, are replaced 

 by teeth, which are attached to the palatine arch, and not to the jaw ; 

 the intestine, which before was long and almost cylindrical, becomes 

 shorter, and is inflated in certain portions of its length ; and the abdo- 

 men, which had been almost spherical, becomes thin and slender: 

 the metamorphosis may now be seen in its entire extent, and more 

 distinctly as regards the locomotive system than any other. 



" The tadpole at first exhibits no trace of either internal or external 

 limbs ; it swims about like a fish by the action of its tail, which is an 

 extensive organ, longer and wider than the body, supported by a pro- 

 longation of the vertebral column, moved by powerful muscles, and 

 supplied with large blood-vessels and numerous nervous branches : 

 beneath the skin and muscles of the anterior and posterior regions of 

 the body, two little projections appear at a certain period; these are 

 the limbs, and are at first attached to the adjacent structures by the 

 nerves and blood-vessels which are supplied to them : these pro- 



