CHAPTER XII 
PHYLUM ECHINODERMA 
Class 1. HOLOTHUROIDEA. Sea-cucumbers. 
>, 2. ECHINOIDEA. Sea-urchins. I SuB-PHYLUM 
5, 3. ASTEROIDEA. Starfishes. J ELEUTHEROZOA. 
>» 4. OPHIUROIDEA. Brittle-stars. 
>, 5. CRINOIDEA. Feather-stars, 
», 6 EDRIOASTEROIDEA. Extinct. SuB-PHYLUM 
3» 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 Ccelen- 
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 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 celomate 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 
