382 '""^ ANIMAL KINGDOM 



groups the slits become U-shaped through the development of tongue- 

 bars. The details of structure in the tongue-bars of hemichordates and 

 chordates are so similar that they become the strongest evidence for 

 relating the groups. Formerly much stress was placed on the possible 

 homology of the stomochord of hemichordates and the notochord of 

 chordates, and on the hemichordate collar nerve and chordate nerve 

 cord, but as more is learned of the details of structure and function in 

 these organs, more doubt is cast on the validity of their homology. Even 

 so, such structures represent similar experiments in evolution, and as 

 such do not argue against a relationship of the two groups. 



The hemichordates, echinoderms and chordates are reminiscent of 

 the annelids, molluscs and arthropods, in which two of the phyla can be 

 related through similarities of development, while another two are re- 

 lated through a comparison of adult structure. These relationships are 

 much more obvious in the annelids, molluscs and arthropods. 



Questions 



1. What characteristics link the hemichordates and echinoderms? The hemichordates and 

 chordates? The chordates and echinoderms? 



2. Characterize the fi\ e living classes of echinoderms. 



3. How do tube feet function? 



4. Describe the skeleton of Asterias. 



5. Compare feeding in sea lilies and sea cucumbers. 



6. What is Aristotle's lantern? 



7. What was unique about the Heterostelea? 



8. Discuss conflicting evidence concerning evolutionary relationships among the Echi- 

 noidea, Asteroidea and Ophiuroidea. 



9. What is a tornaria? 



Supplementary Reading 



The Invertebrates, vol. W, by L. Hyman is devoted to the Echinodermata, and con- 

 tains the same comprehensive and richly illustrated treatment found in her other vol- 

 umes. The phylum is treated thoroughly in Schrock and Twenhofel, Principles of Inverte- 

 brate Paleontology, where the extent of the fossil record is revealed and problems of 

 evolutionary relationship are discussed. 



