346 



Descriptive Zoology. 



GENERAL CHARACTERISTICS OF ECHINODERMATA. 



1 . The body and its various organs are radially arranged. 

 But many show more or less bilateral symmetry. 



2. In their development they all undergo a marked meta- 

 morphosis, the young being bilaterally symmetrical, and 



only in a later stage acquiring 

 the radiate arrangement. 



3. The surface has an exoskel- 

 eton of calcareous plates, with 

 movable spines. 



4. There is a well-developed 

 digestive tube, distinct from the 

 body cavity. 



5. There is a peculiar system 

 of water tubes by which tube feet 



are extended and locomotion effected in the 

 free forms. 



6. They reproduce by means of eggs, and 

 do not bud. 



7. They do not occur in colonies. 



8. They are all rather sluggish. 



9. They are all, without exception, 

 marine. 



10. They have remarkable power of 

 regeneration after mutilation. 



The echinoderms were formerly 

 classed with the coelenterates on ac- 

 count of the radial arrangement of the 

 parts of the body ; but the echinoderms differ sharply 

 from the coelenterates in having a digestive tube distinct 

 from the body cavity, and in having a much higher devel- 

 opment, as shown in the variety and perfection of their 



FIG. 195. STONE LILY 



(CRINOID). 

 From Packard's Zoology. 



