THE ECHINODERMATA 133 



pared with thaf of the Starfish or Sea-Urchin is 

 upside down. The young of the Feather-Stars 

 have stalks by which they are fixed, like the En- 

 crinites; but afterwards the stalk is lost. 



Among fossil Echinoderms there are two 

 groups of stalked forms which have no living 

 representatives. These are the Cystoidea and 

 the Blastoidea. In both of these the stalk bears, 

 as in Encrinites, a calyx or head, which is com- 

 parable, with the body of the free Echinoderms. 



The Sea-Urchins possess a swimming larval 

 stage, which goes through remarkable changes 

 after passing out of the two-layered (Gastrula) 

 form. It becomes provided with cilia, which are 

 arranged in bands, and outgrowths of peculiar 

 form are established in the case of the Sea-Ur- 

 chins, while the larvae of the other groups also pre- 

 sent characteristic shapes. Within the larva the 

 adult form develops, the outside of the larva 

 being finally thrown off. 



In the young Feather-Star, a subsequent stage 

 of the young animal has a stalk, by which, like the 

 Encrinite, it is fixed. This animal therefore is at 

 firstfree-swimming,afterwards fixed, and again free 

 in its final stage a remarkable series of changes. 



These queer-shaped things, the Sea-Urchins 

 and their allies, are perhaps the last creatures 

 amongst which we should think of looking for 

 relations of the Worms. Yet the earliest stages 

 of the larva are considered to present a certain 

 amount of resemblance to the Wheel-ball larva, 

 which has been referred to elsewhere (pp. 42 and 

 72). Still more startling fact, these larvae have 

 been compared to that of alanoglossus, the lowest 

 member of the Chordata, and a relation of the 

 Vertebrates themselves (see p. 143). 



