CHAPTER XII. 



PHYLUM ECHINODERMA. 



Class i. HOLOTHUROIDEA. Sea-Cucumbers. "] 



,, 2. ECHINOIDEA. Sea-Urchins. I SUB-PHYLUM 



,, 3. ASTEROIDEA. Star-fishes. I ELEUTHEROZOA. 



,, 4. OPHIUROIDEA. Brittle-stars. J 



,, 5. CRINOIDEA. Feather-stars. "| 



,, 6. EDRIOASTEROIDEA. Extinct. I SUB-PHYLUM 



,, 7. BLASTOIDEA. Extinct. JPELMATOZOA. 



,, 8. CYSTIDEA. Extinct. 



IN contrast to the " worms," the Echinoderms form a well- 

 defined series. They may be described as sluggish marine 

 animals, generally with superficially radial symmetry, with a 

 tendency to form limy skeletons. The radial symmetry led 

 the older zoologists to place the Echinoderma near Ccelen- 

 tera, but there seems to be no real affinity. More- 

 over, the larval Echinoderm is bilateral in its symmetry. 

 It seems likely that the Echinoderms represent an offshoot 

 of some " worm " stock. As in Ccelentera, the nervous 

 system shows a marked absence of centralisation, which 

 may be connected with the absence of a definite head 

 region, and this again with the sedentary or sluggish 

 habit. 



GENERAL CHARACTERS. 



The Echinoderms are automate marine animals in which 

 the bilateral symmetry of the larva is replaced hi the adult 

 by more or less marked radial symmetry. In addition to 

 the dominant radial symmetry, the adults show to a varying 

 extent a tendency towards the bilateral type, but this is never 

 the same as that of the larva, nor is it equivalent in the 

 different forms. Lime is always deposited in the mesodermic 



