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 Europe 

 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 inarine animals. It is probable that all other sorts of 

 Echinoderms are descended from ancestors resembling the sea 

 lilies, 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 xipon 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 holothurian"'^ 

 swims through the water, and was at first mistaken for a jellyfish. 

 Most of them cast their eggs out into the water, and the larvse 

 develop bands of waving cilia, which enable them to swim about 

 for a considerable time. Suddenly the body of the Echinoderm 

 begins to develop Avithin 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 forhesii 

 extends along the coast from the Gidf 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, to^^gh, spiny skin, and the bright orange madre- 

 poric plate. 



In the North it is replaced by another form called Asterias vid- 



'^' Pelagoihu) la tiatat}ix, Jleuioirs Museum Comparative Zoology at Harvard, Vol. XVIf, No. 

 3, 1894, Plate XIX. 



