ANIMAL S WITH SPINY SKINS— THE ECHINODERMS 289 



there has been formulated a theory of the echinoderm ancestry of the 

 chordates. This theory is quite plausible in spite of the great diversity in 

 the adults of these two phyla. 



A word of caution about the reliability of embryos as a means of es- 

 tablishing animal relationships should be introduced. In 1828, the great 

 German embryologist, Von Baer, brought forth his observations on the 

 similarities among the embryos of different animals. He emphasized the 

 point that the embryo of one animal would resemble the embryo of a 

 closely related animal as we have pointed out. Less scientific workers, 

 such as Ernst Haeckel, seized upon this idea and formulated a "theory 

 of recapitulation" which received wide acceptance for a time. This theory 

 holds that an embryo tends to retrace all the changes which have occurred 

 in the ancestry of the animal. As this theory has been subjected to careful 

 scientific analysis, it has become apparent that this is not true. There are 

 a few similarities in the developing embryos of higher animals and the 

 adults of some more primitive forms, but it is now believed that these 

 are incidental to the development of the embryo and not a recapitulation. 

 The embryo tends only to reproduce those embryonic characteristics 

 which it had in the past as an embryo and not the characteristics of the 

 past adult forms. Thus, comparative embryological studies are of great 

 importance in establishing animal relationships, but not in showing the 

 changes undergone by the adults in their evolutionary development. 



The Starfish — Asteroidea 



The common starfish, Asterias, is neither star-shaped nor is it a fish, 

 but it was given this name when it was thought that stars had five points 

 and when every animal which lived in the water was a fish. We now 

 know that stars are spherical in shape and that fish are only one type 

 of many of the living inhabitants of our ponds, lakes, rivers, and seas; 

 but the name, starfish, is still used for this interesting echinoderm. 



When a starfish is examined it will be noticed that it has radial 

 rather than bilateral symmetry. This might lead to the conclusion 

 that this is a rather primitive animal, about on the level with the 

 coelenterates ; yet, as we study its various systems, it becomes evident 

 that it is much more highly organized than the coelenterates. Some 

 clue to this unusual combination of characteristics is found in the larva 

 of the starfish — the larva is bilaterial in symmetry. Since, as we have 

 just learned, the larva tends to continue developing in the same way 

 after the adult has become different through specialization, we may 

 conclude that this animal was probably bilateral as an adult at some 

 time in its past history. 



