CHAPTER XII 



PHYLUM ECHINODERMA 



Class I. HoLOTHUROiDEA. Sea-cucumbers. 



2. EcHiNOiDEA. Sea-urchins. | Sub-Phylum 



3. AsTEROiDEA. Starfishes. TEleutherozoa. 



■] 



4. Ophiuroidea. Brittle-stars. 



5. Crinoidea. Feather-stars. ^ 



6. Edrioasteroidea. Extinct. I Sub-Phylum 



7. Blastoidea. Extinct. | Pelmatozoa. 



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 Coelen- 

 tera, but there seems to be no real affinity. Moreover, the 

 larval Echinoderm is bilateral in its symmetry. It seems 

 likely that the Echinoderms represent an ofi"shoot of some 

 *' worm " stock. As in Coelentera, 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 coelomate marine animals in which 

 the bilateral symmetry of the larva is replaced in 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 

 tissues {mesenchyme), and in consequence there is frequently a 



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