EVOLUTION OF THE ECHINODERMS — FELL 



461 



Further, since the tornaria larva of enteropneusts closely resembles 

 the auricularia, it was inferred that hemichordates arose from the 

 same hypothetical auricularia ancestry as led to holothurians and 

 asteroids. This necessarily implies a closer relationship between 

 ophiuroids and echinoids than occurs between ophiuroids and aster- 

 oids, an inference so divorced from other evidence as to make the 

 theory unattractive to students of echinoderms, few of whom have 

 supported such speculations. 



In 1948 attention was focused on some obvious weaknesses in the 

 embryological theory. One of them is illustrated in figure 3, which 

 shows how quite divergent types of larval development can occur 

 within single groups, in this case ophiuroids. Some have a pluteus 

 larva, others have a vitellaria or yolk-larva, similar to the larva of 

 crinoids, and others again have no larva at all, not even a vestige of 

 one. Thus, differences in mode of development do not necessarily 

 indicate different origins, for obviously all ophiuroids are more 

 closely interrelated to one another than to other echinoderms. 



Figure 4 illustrates another criticism; quite unrelated assemblages 

 can share common larval forms. Here we see diagrammatic life 

 histories of a crinoid (on the right), an ophiuroid (on the left), and 

 a holothurian (above), so arranged as to bring the common larval 

 form, the vitellaria, into proximity. Obviously none of these can be 

 closely related, yet all share a similar larva. On the basis of tliis 



Young 

 ophiuroid 



Gastrulae 



Figure 3. — Divergent types of development in the life histories of related echinoderms 



(Ophiuroidea). 



