20 



Animals with Spiny Skins — The 

 Echinoderms 



This phylum name, Echinodermata, means spiny skin; and, if you have 

 seen a starfish, or perhaps stepped on a sea urchin at the seashore, you will 

 realize that the name is a very appropriate one. Most members of this 

 phylum have a skin which has become calcified and, therefore, is quite hard 

 and brittle with numerous spines which are protective in nature. They are 

 all marine and, as a result, not well known to those not living near coastal 

 regions. They are very abundant in salt water, especially in tropical 

 waters. 



The echinoderms at first glance are difficult to fit into an orderly series 

 of invertebrate phyla. In each of the previous phyla, with the possible 

 exception of the mollusks, the typical animal selected for study possesses 

 just about everything that the preceding one had, plus some additional 

 characteristics that make it a more advanced type of animal. The echino- 

 derms are fairly advanced in some systems, but in others they seem more 

 like some of the simplest metazoans, especially the coelenterates. In still 

 other systems they are neither primitive nor advanced, but specialized in 

 ways that are peculiar to themselves. However, when their embryology 

 is studied critically, we find many characteristics which are more like those 

 of the chordates than any of the invertebrate phyla. For example, they 

 form their three germ layers and their coelom in almost exactly the same 

 way as Amphioxus, which is a primitive chordate animal. This is of 

 particular significance because many studies have shown that embryonic 

 characteristics are usually more reliable than adult characteristics in de- 

 termining animal relationships. Adults often become so highly specialized 

 in adaptations to varying environments that they may show little resem- 

 blance to the adults of rather close relatives which may have become 

 specialized along other lines. The embryos, however, tend to start de- 

 velopment in somewhat the same manner as they did before the adults 

 become so specialized. Hence, when we find two animals with a veri- 

 similar method of embryonic development we may assume that these two 

 animals are rather closely related, even though they may show considerable 

 morphological difference as adults. Because of the similarities in the 

 embryonic development of the echinoderms and the primitive chordates, 



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