METAZOA MOLLUSCA. 263 



embryonic stage only, which is quickly lost, so that the 

 adult is without even a vestige of a protective covering. 



The marine Gastropods probably gave rise to the 

 specialized fresh-water and terrestrial species, transi- 

 tional forms existing at the present time. 



The Cephalopoda illustrate acceleration in development 

 and specialization in structure. The external, chambered 

 shell becomes internal and extremely modified ; it may 

 also exist as a vestige or be wanting altogether. 



