CHAPTER XII. 



ECHINODERMA. 



Class I. HOLOTHUROIDEA (Scytoderma). Sea-Cucumbers. 



2. Echinoidea. Sea-Urchins. i - 



3. Asteroidea. Star-fishes. LEuwthkrozoa 



4. Ophiuroidea. Brittle-stars. J or Echisozoa. 



5. Crinoidea. Feather-stars. \ 



6. Blastoidea. Extinct. [-Pelmatozoa. 



7. Cystoidea. Extinct. J 



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 the larval Echinoderm is more specialised than 

 most of the larval " worms," and is bilateral in its symmetry. 

 It seems likely that the adult radial symmetry is an adapta- 

 tion to sedentary life, and 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 relatively sedentary habit. 



General Characters. 



The Echinoderms include forms in which the bilateral 

 symmetry of the larva is replaced in the adult by radial 

 symmetry. In addition to the dominant radial symmetry, 

 the adults show to a varying extent a tetidency 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 iti con- 



