THE PHYLA HEMICHORDATA AND ECHINODERMATA 



377 



closely (Fig. 19.19). In two different chemical analyses, one on sterols and 

 one on phosphagens, the ophiuroids differed from the asteroids and 

 were identical with the echinoids whereas the asteroids resembled the 

 holothurians and crinoids. These embryologic and chemical studies sug- 

 gest that the ophiuroids are more closely related to the echinoids than 

 to the asteroids. 



Obviously both theories cannot be correct. The echinoids existed 

 long before the ophiuroids, so that the ophiuroids cannot be both closely 

 related to the echinoids and yet descendants of the asteroids. Other 

 chemical studies, which are still in progress, tend to negate the evidence 

 from the sterol and phosphagen analyses. 



1 73. Relationships among the Hemichordata, Echinodermata, 



and Other Phyla 



From the study of adult structure given in this chapter there is little 

 evidence for relating the hemichordates and echinoderms. Both have a 

 poorly developed nervous system with few sense organs, a negative char- 

 acteristic that can also be found in other animal groups. In both groups 

 a portion of the coelom opens to the outside, filling that portion with 

 sea water to serve as a hydraulic mechanism. 1 his is most remarkable, 

 since such a device is not found in other animals. The similarity, how- 

 ever, appears to be functional rather than structural, for the adult 

 morphologies of the two mechanisms are very different. 



-Anus 



E F 



Figure 19.17. Diagrammatic representation of early development in the hemichor- 

 dates and echinoderms. The lower figures are sections through the embryo indicated by 

 arrows in B, and show two different methods of coelom formation, both of which are 

 found in both phyla. 



