§2 A.V INTRODUCTION TO ZOOLOGY. 



Animals which have not the power of moving from 

 place to place are often possessed of a radial sym- 

 metry, consisting of similar parts arranged with refer- 

 ence to a centre, like those of a flower. The radial 

 symmetry, is in fact the expression of a fixed 

 position : the whole symmetry of the majority of 

 plants is essentially radial, though mostly modified 

 into a spiral type by the successive development 

 of their parts in chronological order : while the 

 occurrence of a bilateral symmetry in them is usually 

 traceable to mechanical conditions, such as reference 

 to surfaces affording support or moisture, aptitude 

 for undulation under the action o£ wind or water, or 

 adaptation to the entrance of insect visitors. 



There are animals which are free but float rather 

 than propel themselves, which, like the fixed ones, also 

 present a radial symmetry: both usually belong to the 

 group called Coelenterata. The same group presents 

 some interesting links between radial and bilateral 

 symmetry among some of its more active free-swim- 

 ming forms. A curious instance of a partial radial 

 symmetry is presented bysome o( the, Echiiiodermata,ioT 

 example, the Star-fishes, which in their larval stages are 

 free-swimming and bilateral, and in their adult stages, 

 though unfixed, move in any direction indiEferently ; 

 the axis of their vertical height approximately repre- 

 sents the longitudinal axis of other animals, so that 

 they move sideways. The Sea-urchins, nearly allied 

 to them, present several curious intermediate types 

 of symmetry. 



