242 PALEONTOLOGY 



pentamerous symmetry. Such a symmetry is common 

 among flowers, but is quite unknown among animals 

 except in the echinoderms. There is one other animal 

 phylum with a radial symmetry the Coelenterata and 

 in Cuvier's classification, now a century old, the two 

 were united as Radiata. But not only is the symmetry 

 of the Ccelenterata a four-, six-, or eight-rayed symmetry, 

 but it is in general a more perfect symmetry than that of 

 echinoderms. The former include some forms, such as 

 the jelly-fish, which are absolutely radial throughout 

 their bodies ; the latter include none such, though some 

 attain almost perfect radiality in their skeleton only. In 

 some, again, radial symmetry has not yet been attained ; 

 in others it has been modified into a practically bilateral 

 symmetry. 



The skeleton of echinoderms is not truly external like 

 that of Brachiopods or most Mollusca ; there are soft 

 living tissues outside it, but these are so thin in most 

 cases that it may be roughly spoken of as external. Still, 

 it is important to remember these external tissues in 

 order to form an idea of the life of echinoderms. 



The Echinodermata fall into two main divisions 

 Pelmatozoa or fixed forms, and Eleutherozoa or freely- 

 moving forms. To be precise, the former are fixed at 

 some time in their life (and usually throughout adult 

 life), the latter never. The former were abundant in the 

 Palaeozoic era, rare afterwards ; with the latter the 

 reverse is the case. They are further divided into 

 classes, thus : 



