178 



CASWELL GRAVE. 



Briefly then, the earliest ancestor of the group of echinoderms 

 of which there is much trustworthy evidence, was a free-swim- 

 ming organism of microscopic size with an elongated body and a 

 long preoral lobe. At the tip of the preoral lobe a sense organ and 



FIG. lo. Young Mellita testiidinata. Original. 3, 4, and 5, ciliated rings. 



nerve center was located. The contours of the body were plain, 

 no arms or processes of any kind being present. The alimen- 

 tary tract occupied the posterior part of the body, the mouth and 

 anus opening ventrally. Three pairs of body cavities, arranged 

 symmetrically with reference to the alimentary tract, were present. 



