THE STAR-FISHES. I 71 



colony of worms. The Sea-lilies, or Crinoida, difter 

 least from them, but having given up the free, slow motion 

 possessed by other Sea-stars, they have become adherent to 

 rocks, etc., and form for themselves a long stalk. Some 

 Encrinites, however (for example, the Comatulse, Fig. B, 

 on Plates VIII. and IX.), afterwards detach themselves from 

 their stalk. The original worm individuals in the Crinoida 

 are indeed no longer preserved in the same independent 

 condition as in the case of the common star-fish ; but they 

 nevertheless always possess articulated arms extending from 

 a common central disc. Hence we may unite the Sea-lilies 

 and Sea-stars into a main-class, or branch, characterized as 

 possessing articulated arms (Colobrachia). 



In the other two classes of Echinoderma, the Sea- 

 urchins and Sea-cucumbers, the articulated arms are no 

 longer present as independent parts, but, by the increased 

 centralization of the stock, have completely fused so as to 

 form a common, inflated, central disc, which now looks like 

 a simple box or capsule without arms. The original stock 

 of five individuals has apparently degenerated to the form- 

 value of a simple individual, a single person. Hence we 

 may represent these two classes as a branch character- 

 ized as being without arms (Lipobrachia), equivalent to 

 those which possess articulated arms. The first of these 

 two classes, that of Sea-urchins (Echinida) takes its name 

 from the numerous and frequently very large thorns which 

 cover the hard shell, which is itself artistically built up of 

 calcareous plates. (Fig. C, Plates VIII. and IX.) The funda- 

 mental form of the shell itself is a pentagonal pyramid. 

 The Sea-urchins probably developed directly out of the 

 group of Sea-stars. The different classes and orders of 



