ECHINODERMATA. 213 



to a fixed or sluggish life. Bilateral symmetry is the symmetry 

 of moving animals, and is regarded as of a higher type. It therefore 

 looks as though the echinoderms deteriorate as to symmetry during 

 their embryonic development. This is taken by some naturalists 

 as meaning that the whole group was once bilateral, and is now 

 really lower in the scale in this respect than its ancestors were. 



While there is probably no power to form new individuals by 

 fission as in the worms, it is known that some starfish may 

 regenerate new arms when these are broken away. It is claimed that 

 an arm may even regenerate a new disc, and thus a starfish torn 

 to pieces may make several new ones. 



^an 



FIG. 67. Diagrams of larval stages in two echinoderms, showing the bilateral con- 

 dition, a, arms; an, anus; m, mouth. 



Questions on the Figure. Enumerate the points showing bilateral 

 symmetry. 



232. Definition of the Echinodermata. There is great difference 

 between the five principal classes of this group, but they are 

 especially noted for the five-fold plan of the body organs, whether 

 they are strictly radial or not; and for the water vascular system. 

 The principal organs have their main portions in this central disc: 

 as the digestive, the nervous, the water vascular, the body cavity. 

 There may be portions passing into the arms, but these are branches 

 of the central part. This brings it about that there is nothing 

 which can properly be called a head. What corresponds to the 

 head of other animals is in the center of the body. 



233. Classification of Echinodermata. 



Stalked; usually attached; mouth turned upward. 



Class Crinoidea. 

 Examples: Sea-lilies; Antedon, Pentacrinus; Metacrinus. 



