CHAPTER XVI 



PHYLUM ECHINODERMATA 



The Echinodermata (e ki no dur'ma ta — hedgehog skin) constitute 

 a rather backward phylum of animals which are thought to have 

 undergone a certain amount of retrogression in structural features. 

 That is, they seem to have a lower level of organization than that 

 possessed by some of their ancestors. The modern echinoderms, as 

 the group is commonly called, possess several distinctive charac- 

 teristics. Some of these characteristics are as follows : skin cov- 

 ered with spines ; lack of segmentation ; triploblastic radial sym- 

 metry, subduing a primitive bilateral symmetry; water vascular or 

 ambulacral system and tube feet ; circumoral nerve ring ajid radial 

 nerves; a calcareous skeleton composed of plates; pedicellariae ; 

 and a coelom. The external opening into the water vascular sys- 

 tem is called the madreporite. It is located on the dorsal or aboral 

 side, at an interradius between arms in such a position that a line 

 drawn through it and on through the radius of the opposite ray, will 

 divide the body into two similar halves. There will be a half ray 

 and two complete rays in each half of a five-rayed animal divided 

 in this way. 



Classification 



In earlier classifications as in the case of Cuvier, this entire group 

 was included along with coelenterates in a group called Radiata, 

 The basis for this was the apparent similarity of radial symmetry. 

 It was later discovered that the coelenterates have a typical primi- 

 tive radial symmetry while the Echinodermata have only a second- 

 ary radial symmetry which is derived from a bilateral condition. 

 This is indicated quite definitely by the fact that the larvae of 

 echinoderms have a typical bilateral symmetry. The change which 

 occurs seems to be an adaptation to a sedentary habit. This phylum 

 is usually divided into five classes of modern forms including such 

 common animals as starfishes, brittle stars, sea urchins, sea cucum- 

 bers, and sea lilies. 



Class Asteroidea. — The general features of the body include a 

 central disc usually with five arms or rays radiating from it. There 

 are some species which do not adhere to this pentamerous condition 



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