42 EVOLUTION OF LIFE. 
stationary condition, rooted by a stem; in this immature 
state it was supposed for a long time to bs a distinct animal, 
known as the Pentacrinus. (Fig. 42.) After a time, how: 
ever, the stem disappears, and the little creature floats off 
as the very pretty Comatula. The Comatula is very inter- 
esting, as its early Pentacrinus stage is the only living 
representative of the Crinoids, known commonly as lily 
stones, and as 5t. Cuthbert's beads, when segments of the 
stem alone are preserved. The Crinoids are now extinct. 
but are preserved in great profusion in the fossil state. As 
the star-fishes in one stage of their existence are more or 
less fixed, and as the Crinoids have died out, save the only 
living example, the young of the Comatula, it is possible that 
the Crinoids are the earliest of the Echinodermata. Haeckel, 
however, considers the Crinoids as a very ancient offshoot 
of the star-fishes, adapted to the fixed state of living. Per- 
haps the Crinoids and star-fishes are the diverging stems 
of an intermediate group, partaking in its nature of the 
peculiarities of both these classes. In either case the star- 
fishes are the progenitors of the sea-urchins, and they of the 
sea-cucumbers. Imagine the five arms of the star-fish bend- 
ing down until their free ends, uniting in the middle, form 
a ball-shaped figure; suppose the empty spaces between 
the arms to be filled up, and a sphere will be formed. Such 
are the relations of the star-fish to the sea-urchin,or Echinus. 
Many intermediate fossil forms have been found connecting 
these extremes. The sea-urchins, or sea-eggs, are covered 
with innumerable spines or bristles; hence their name of 
Echini. (Fig.44.) These spines are movable, being loosely 
ER 
articulated to little knobs covering the body. When one 
watches a living Echinus, there may be seen protruding 
between the spines sucker-like appendages, which serve as 
a means of progression. If the spines be removed, the 
body of the Echinus is seen to be a hollow sphere, com- 
posed of arms (ambulacral plates) and intermediate arms 
