METAZOA COELENTERA. 143 



a great variety in form, but through this extreme diversity 

 the fundamental type of structure remains the same. 



The subkingdom of Vermes or Worms is placed next 

 the Coelentera by many authorities, and the larvae of cer- 

 tain worms are considered as the nearest approach to the 

 ancestral forms from which all the remaining invertebrates 

 and also the vertebrates have descended. 



There is a resemblance, speaking broadly, between the 

 larvae of echinoderms, molluscs, and worms, but this sim- 

 ilarity may be due to inheritance from some pre-Cam- 

 brian ancestor from which the three branches have devel- 

 oped along different lines. When one considers the 

 varied and extreme specializations of worms ; the articu- 

 late plan of structure differing so essentially from the ra- 

 diate plan ; the greatly developed muscular system and 

 the complex excretory and reproductive organs ; the large 

 number of extremely reduced forms, one finds it easier to 

 place the worms among the more specialized and the ar- 

 ticulated animals than next to the comparatively simple 

 Coelentera. 



The Echinoderms, on the other hand, are pre-eminently 

 radiate organisms, and in many ways they possess char- 

 acters in common with the Coelentera. 



