CHAPTER XII 



ECHINODERMA. 



Class I. HOLOTHUROIDEA (Scytoderma). Sea Cucumbers. 



2. ECHINOIDEA. Sea Urchins 



3. ASTEROIDEA. Starfishes 



4. OPHIUROIDEA. Brittle stars 



5. CRINOIDEA. Feather stars "I 



6. BLASTOIDEA. Extinct VPELMATOZOA. 



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 of radiate symmetry, with a 

 tendency to form limy skeletons. The radial symmetry led 

 the older zoologists to place the Echinoderma near 

 Ccelentera, 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 adaptation to sedentary life, and that the Echinoderms 

 represent an offshoot of some "worm" stock. Yet it is 

 interesting to notice that in both Ccelentera and Echino- 

 derma 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 tendency towards a bilateral form, but this is 

 never the same as that of the larva, nor is it equivalent 

 in the different types. Lime is always deposited in the 



