ECHINODEEMATA. 41 



(5) the blastula, a spherical mass of cells resembling a 



blackberry ; (It is really a hollow sphere.) 



(6) the gastrula, a cup-shaped structure, formed as if 



one side of the blastula had been pushed in ; 



(7) the larva, a transparent bi-lateral creature with sev- 



eral "arms," which swims about freely. (Dem.) 

 The first six stages illustrate the development (em- 

 bryology) of the many-celled animals (metazoa). 



