ANIMAL METAMORPHOSIS. 185 



has proceeded in a retrograde direction, tending rather to a 

 process of degeneration than to one of advance. This has been 

 ably brought to notice by Dr. Dohrn, of Naples, and by Prof. 

 Ray Lankester. We have thus been enabled to determine the 

 affinities of several creatures which we should not otherwise have 

 dreamt of if we had only studied the adult animals. Nothing at 

 first sight could be more unlike than a shrimp and a barnacle, or 

 a lively, free-swimming Entomostracon and a parasitic Lernseocera 

 or Sacculina ; but the fact that all, in their early stages, pass 

 through the active Nauplius form shows that they are all equally 

 Crustaceans, some of which have proceeded to a more advanced 

 development, while others (especially those which have taken on 

 a parasitic condition) have lost their organs of sense and motion, 

 and retrograded in the scale of existence. 



Finally, metamorphosis has been the means of introducing us 

 to a far-away series of our ancestors in the remote past, going 

 back far beyond primseval man and the anthropoid apes. The 

 discovery of Kalewsky, that the tadpole-like larva of the Ascidians? 

 or sea-squirts, possesses in its caudal appendage a cartilaginous 

 axial rod which is homologous with the primitive backbone or 

 notochord, has led certain observers to conclude that the Asci- 

 dian mollusc is not only the original ancestor of all the Verte- 

 brata, but even of man himself. Haekel goes still further back, 

 and concludes that " in the gastrula is now found the common 

 ancestral group from which all tribes of animals can, without 

 difficulty, be derived. It is one of the most ancient and import- 

 ant ancestors of the human race." 



EXPLANATION OF PLATE XVII. 



Fig. 14. — Development of Mussel : — 



1. Glochidium, Larva still within the Egg. 



2. Shell of Glochidium, widely opened. 



3. Glochidium, viewed from the side, showing hooks 



h. b. 



5, 15. — Very young Oysters. 



,, 16. — ZojTea of Crab, more magnified than Fig. 17. 



,, 17. — Early stage of Crab, with tail. 



