METAZOA ECHINODERMA. 195 



The Holothuroidea like the Echinoidea have no fixed 

 stage in their development. As has been said, they 

 resemble the more specialized invertebrates in being 

 bilaterally symmetrical and in having an antero-posterior 

 axis of the body. They possess many reduced characters ; 

 the number of feet is limited, and the water-vascular 

 system in some forms is a mere ring around the mouth. 

 This reduction is indicative of specialization and it offers 

 a reason for placing this group farthest from the primitive 

 ancestral form of Echinoderms. 



