3i8 



PHYLUM ECHINODERMA 



characteristic of the Trochosphere. The fact that this is represented 

 in the larva of Antedon is regarded by many naturalists as a point of 

 much importance. 



Relationships of Echinoderma 



The Echinoderms form an exceedingly well-defined phylum, but 

 the Holothurians especially show how many of the significant char- 

 acters may be lost. In that class we see how the power of forming a 

 calcareous skeleton, the characteristic tube-feet, and the greater part of 

 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 Cystidea 

 and the Eleutherozoa. 



