Metazoa A mphioxus. 2 5 1 



CHAPTER X. 



METAZOA VERTEBRATA. 



SECTION I.AMFHIOXUS. 



IT has been already seen that it is possible to subdivide the 

 Animal Kingdom into the groups Protozoa and Metazoa, and 

 that the Metazoa may be again divided into those with and 

 those without a vertebral column Invertebrata and Verte- 

 brata, or more correctly Achordata and Chordata. Of the 

 former group we have examined two representatives, Obelia 

 geniculata and Lumbricus terrestris. These two forms may 

 be taken to represent the great classes Hydrozoa and 

 Vermes respectively. But between these and the Chordata 

 there come an immense series of forms, represented by such 

 animals as the sea-urchin and starfish (Echinodermata) ; 

 snail, mussel, and cuttlefish (Mollusca) ; cockroach and bee 

 (Insecta) ; lobster and crab (Crustacea) ; and many others 

 less familiar. To enter into the discussion of these groups 

 would necessitate the study of many different types, greatly 

 exceeding the limits of this text-book. 



We may pass, however, from the study of the earthworm 

 to that of the Chordata the more easily and naturally inas- 

 much as zoologists believe that that group had in far past 

 ages worm-like ancestors. In discussing the structure of 

 the nervous system of Lumbricus it was noted that the gene- 

 rally diffused nervous system in the lower types, consisting 

 of nerve-fibres and nerve-cells, had become aggregated, 

 first, into two lateral cords, and then, by approximation, 



