PLANTS AND ANIMALS 



213 



Aristotle and other ancient writers, it had 

 been observed that sponges grow attached 

 firmh' to rocks by root-like extensions, yet 

 possess some feeling, that sea-cucnmbers 

 (Holothuria) and sea-slugs (Ctenophora), 

 although having 

 freedom of move-*^i 

 ment, seem to lack 

 feeling, and in this 

 respect behave 

 like plants, that 

 sea-anemones are 

 fixed objects, 3^et 

 are sensitive like 

 animals, and that 

 man\' other living 

 things have char- 

 acteristics that 

 make them in like 

 manner uncertain 

 of classification. 

 Aristotle's conclu- 

 sion from these 



facts, that ^'na- ^^^- '^^- — '^ sea-cucumber with 



. ex tended branched tentacles. 



ture passes grad- (After ciaus.) 

 ually from the insentient over to the sentient 

 through forms that trulj^ live but are not 

 animals, " only nc^ds the substitution of 

 Mycetozoa and some Flagellata for the Po- 



