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Figure 19.16. An extinct class, Heterostelea, view of upper side. This is one of sev- 

 eral bilaterally symmetrical genera of early Paleozoic echinoderms. (After Bather.) 



bilaterally symmetrical echinoderms of the early Paleozoic. These were 

 attached by a stalk like the crinoids, but apparently held the body in a 

 horizontal position. All the other echinoderm classes have radial sym- 

 metry. 



Three more of the extinct classes were attached by stalks like the 

 crinoids. All of these attached forms (including the feather stars) are 

 placed together in the subphylum Pelmatozoa. Within this subphylum 

 the bilateral symmetry of the Heterostelea gave way to the radial sym- 

 metry of the other classes, presumably as an adaptation to an attached 

 existence. Except for the feather stars, which are attached only when 

 young, the Pelmatozoa appear to be on the verge of extinction. 



The unattached echinoderms, which include the Holothuroidea, 

 Echinoidea, Asteroidea, Ophiuroidea and one extinct class, are placed in 

 the subphylum Eleutherozoa. In a few species of starfish the larva at- 

 taches to the bottom briefly during its metamorphosis into the adult 

 form, but in most asteroids and in all other eleutherozoans that have 

 been studied, the individuals are never attached. While it is generally 

 concluded that the Eleutherozoa evolved from the Pelmatozoa, it is not 

 known whether they evolved once or whether some of the classes arose 

 separately from attached forms. The Eleutherozoa apparently did not 

 evolve from the Crinoidea, but arose from some extinct and possibly 

 unknown pelmatozoan. 



Evolutionary relations among the four living classes of the Eleuth- 

 erozoa are obscure. A comparison of the adult anatomy suggests that the 

 Asteroidea and Ophiuroidea are the most closely related, and that the 

 Echinoidea and Holothuroidea form two distantly related groups. 

 The fossil record supports this arrangement. The Holothuroidea, Echi- 

 noidea and Asteroidea are found as fossils in the early part of the 

 Paleozoic Era, 350 million years ago. The Ophiuroidea begin as fossils 

 only 275 million years ago. During the 75 million years between the first 

 asteroids and the first ophiuroids there were a number of species inter- 

 mediate in morphology between these two classes. They can be arranged 

 in a series suggesting many steps in the evolution of the Ophiuroidea 

 from asteroid-like ancestors, particularly in the skeletal modification of 

 the rays. 



Fossil evidence as convincing as this for the origin of a class is rare, 

 and should be conclusive. Other evidence, however, contradicts the con- 

 clusion that ophiuroids are close to the asteroids. Ophiuroid larvae are 

 different from those of the Asteroidea, but resemble echinoid larvae 



