RELATIONSHIPS OF ECHINODERMA. 279 



the peculiar water- vascular system, may all disappear ; it is conceivable 

 that further modification of the same kind might eliminate all the dis- 

 tinctively Echinoderm characters, and produce an organism whose 

 systematic position would be very difficult to determine. This is 

 important, because, as we have already seen, there are many "worm- 

 like " types of whose affinities we know nothing. That some of these 

 are related to Echinoderms has been often suggested. 



It is conceivable that Holothurians of the worm-like Synapta type 

 are nearest the primitive stock of Echinoderma. But there are strong 

 arguments in favour of the view that the free forms, the Eleutherozoa, 

 have been derived from attached Pelmatozoic ancestors. The extinct 

 Edrioasteroidea are in some ways intermediate between the Cyst idea 

 and the Eleulheiuzoa. 



