THE NEW EVOLUTION 



Also exclusively marine are the echinoderms — the 

 starfishes (fig. ^z, p. 71), brittle-stars (fig. 44, p. 87), 

 sea-urchins (fig. 41, p. 71), sea-cucumbers and sea- 

 lilies (fig. 6, p. 5) and feather-stars (fig. 43, p. 87), 

 or crinoids. There are many hundreds of different 

 kinds of these, and they are found from between tide 

 marks down to the greatest depths at which animal 

 life exists. They are relatively most important in the 

 deeper portions of the seas, but on all coasts they are 

 among the most familiar and characteristic of sea 

 animals. A single type of curious sea-cucumber (Pela- 

 gothurid), which is shaped more or less like an um- 

 brella, lives floating freely in the sea some distance 

 beneath the surface. All other echinoderms are bot- 

 tom livers. 



The last group of animals which is exclusively 

 marine includes the ascidians or tunicates. Some of 

 these, as the sea-peaches and the sea-squirts, are 

 familiar to every fisherman. Like the echinoderms, 

 the tunicates live from the shore line down to the 

 deepest portions of the seas; but in contrast to the 

 echinoderms many different kinds, such as the salps 

 (fig. 58, p. Ill), pyrosomas (fig. 59,p. III)andappen- 

 dicularians (fig. 56, p. Ill), live suspended freely in 

 the water. 



The great differences and at the same time the curi- 

 ous correspondences between the animals of the land 

 and the creatures of the sea are important to remember 

 in considering the problem of the development of 

 animal types. 



[72.] 



