CHAPTER IV 

 ECHINODERMATA 



This phylum consists of a number of well-known forms all of which 

 are marine and most of which are very slow in their motions. As their 

 name indicates they are the "spiny-skinned" animals, though the 

 spininess varies greatly in different species. They are radially sym- 

 metrical and were, therefore, by earlier writers classed with the 

 Coelenterates as "Radiata." Their radiate structure is usually built 

 on a plan of five, and their organs of locomotion, in most cases, consist 

 of small sucker-like tube feet, controlled by a characteristic system 

 of water tubes known as the water-vascular system. Unlike the 

 ccelenterates they have a well-developed coelom and possess the three 

 cell layers characteristic of the higher animals. An anus, distinct 

 from the mouth, is generally present. 



There are many extinct groups of echinoderms that, as fossils, are 

 interesting and important geologically. 



The living representatives of this phylum are usually divided into 

 five classes, three of which are of some, though not of very great, 

 economic importance, the other two classes are of scientific interest 

 only. 



I. Asteroidea, Fig. 13. As their name indicates these are the 

 starfishes, the most familiar of the echinoderms. They are typically 

 five-rayed, the rays meeting in an indefinite central disk. On the 

 upper or aboral side is a generally distinct madreporic plate, on the 

 lower or oral side is seen the centrally located mouth, radiating from 

 which are the five ambulacral grooves, filled with tube feet. 



II. Echinoidea, Fig. 14, the sea urchins. The pentanerous ar- 

 rangement is not so obvious here as in the preceding class since there 

 are no radiating arms. The calcareous plates are united into a more 

 or less globular shell or test, Fig. 15, covered with movable spines 

 (hence the name, hedgehog), and perforated with five radiating sets 

 of holes for exit of the long, slender tube feet. The plates that bear. 



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