ECONOMIC IMPORTANCE OF THE GROUP 173 
starting the growth and development of the starfish larva. 
The larva is at first wholly unlike the starfish in form; there 
is not even a suggestion of the starfish outline. After the 
larva has attained a certain size a tiny bud appears which 
develops into the little 
starfish. The bud de¬ 
velops on the larva and 
feeds on it. From this 
bud the adult starfish de¬ 
velops. This is another 
example of metamorpho¬ 
sis (Figure 160). 
157. Other Echino- 
derms. — The sea-urchins 
are thickly covered with 
spines and have tube feet 
which, in many cases, 
may be greatly extended. 
When the spines are re¬ 
moved, an exoskeleton is 
revealed, which plainly 
shows the radial arrange¬ 
ment characteristic of the 
echinoderm group. 
158. Economic Impor¬ 
tance of the Group. — 
Of echinoderms the star¬ 
fish alone has an economic 
bearing. It is harmful. 
Living as it does in the 
region of the oyster and clam beds and feeding almost ex¬ 
clusively on them, the starfish annually destroys thousands 
of dollar worth of clams and oysters. By removing the sea¬ 
weed where the immature starfish gather and by dragging the 
oyster and clam beds great numbers of starfish are destroyed. 
Figure 162. — The Common Sea- 
cucumber. 
An Echinoderm without any skeleton 
for protection. It has become adapted to 
living in the sand much like the worms. 
