THE STAR-FISHES. 171 
colony of worms. The Sea-lilies, or Crinoida, differ 
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 Comatule, 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 (Kchinida) 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 eof Sea-stars. The different classes and orders of 
