TRANSFORMATION OF ORGANS OF ANIMALS 41 



The antennule (fig. 1 6) consists of a protopodite 

 (pr.), furnished with an annulated endopodite (en.) 

 and exopodite (ex.). 



The eye-stalk (fig. 17) consists of a two-jointed 

 protopodite (pr.). This is all that remains, the 

 endopodite and the exopudite being 

 absent altogether. 



This examination of the appen- _ , t 



FIG. 17. Aslacus fluvia- 



dages of the cray-fish clearly shows f! 7 *f- Left eye-staik 



* * (1,5/1) : pr, protopodite 



'that all fresh adaptation in the (Huxiey). 

 appendage entails the modification of some parts 

 and the degeneration of others. In each case 

 evolution is accompanied by degeneration. 



3. Transformation of homologous organs in 

 individuals of different species. 



The limitations of our present knowledge make 

 it difficult to determine definitely the origin of 

 limbs among vertebrates, but they are universally 

 supposed to have developed from the lateral folds 

 which still persist in Amphioxus, and which pro- 

 bably existed in the ancestors of vertebrates. 1 



1 A. Morphological proofs : 



(a) Lateral folds of amphioxus. 



(b) Identity of the skeletons of paired and unpaired fins. 



(c) The number of spinal nerves passing to the fins. 



(d) The mode of entrance of these nerves into the fins. 

 B. Embryological proofs : 



(a) Continuous lateral folds in the embryoes of fish. 



(b) The formation of metameric pouches (coelomic in- 



vaginations not only at the point of origin of the 

 limbs, but between them). 



