STARFISHES, URCHINS AND SEA-CUCUMBERS 51 
seaweeds from the rocks. Some species can even gnaw away the 
rock itself, and in many parts of the world we find that the sea 
urchins have literally honey-combed the rocks; indeed we often 
find a sea urchin living in a cavity whose opening is too small to 
allow of the animal’s escape. The common sea urchin of Hurope 
is sold in the markets during the season when it is full of eggs. 
The sea lilies or Crinoidea are now among the rarest and most 
graceful of marine animals. It is probable that all other sorts of 
Echinoderms are descended from ancestors resembling the sea 
liles, for in long past ages they were far more abundant than any 
other Echinoderms, and even in the age of the chalk they lived in 
countless numbers in shallow water along our shores, their flower- 
like bodies mounted upon long delicate stems that formed veritable 
forests beneath the sea. The sea lily is not a plant, however, but 
may be compared to a starfish mounted upon a long stem which 
arises from the middle of its back and anchors it to the bottom of 
the sea. The mouth is turned upward, and is surrounded by 
branching arms which sweep gracefully to and fro in search 
of prey. 
The Echinoderms live only in salt water, but they are found 
at all depths and in all oceans, from the Tropics to the Poles. The 
vast majority crawl over the bottom, but at least one holothwrian* 
swims through the water, and was at first mistaken for a jellyfish. 
Most of them cast their eggs out into the water, and the larve 
develop bands of waving cilia, which enable them to swim about 
for a considerable time. Suddenly the body of the Echinoderm 
begins to develop within the larva, and most of the old larval 
body is absorbed or cast off. 
The Common Starfish, (Figs. 24, 25). There are two well 
marked varieties of the common starfish ; one called Asterias forbesi 
extends along the coast from the Gulf of Mexico to Massachusetts 
Bay but is rare north of Cape Cod. It is found from low tide level 
to a depth of 120 feet, and may be recognized by its somewhat 
blunt-tipped arms, tough, spiny skin, and the bright orange madre- 
poric plate. 
In the North it is replaced by anotber form called Asterias vul- 
© Pelagothuria natatvix, Memoirs Museum Comparative Zoology at Haryard, Vol. XVII, No. 
3, 1894, Plate XIX. 
