CHAPTER XI 

 STRUCTURE AND CLASSIFICATION OF ECHINODERMS 



(ECHINODERMATA) 



This is one of the most sharply-marked phyla in the animal 

 kingdom, having specialized on very distinctive lines. Though 

 considered after the flat-worms, it must not be imagined that it 

 occupies a lower place in the scale. Indeed, as explained earlier 

 (p. 1 1 ), it is impossible to place the various groups of animals in 

 a linear series, each member of the series higher than the one next 

 below it, and lower than the one next above it. But it is clearly 

 necessary to describe the groups one after the other, just as if they 

 actually formed such a series, and this is apt to give misleading 

 ideas. 



The phylum contains marine animals only, and probably the 

 most familiar of these is the Common Star- Fish (Uraster rubens), 

 found in abundance on many parts of our coast. It will serve 

 very well as our illustrative example. 



The body of a Star-Fish (fig. 277) is built on a plan quite 

 different from those exhibited by the bilaterally symmetrical 

 animals with which we have so far been concerned. As in a very 

 large number of marine Invertebrates, there is a free-swimming 

 larva, known in this case as a Bipinnaria, which is markedly 

 bilateral. It is converted into the adult stage by a very com- 

 plicated series of changes, constituting a true metamorphosis, one 

 result of which is that the original bilateral symmetry becomes 

 obscured, being replaced by a simpler kind of regularity to which 

 the name of radial symmetry has been given, and which is well 

 exemplified by the flower of a primrose, a cart-wheel, or a regular 

 pentagon. In one kind of Star- Fish, indeed (Goniaster), the body 

 is a pentagonal disc, but in the form selected for description, the 

 corners of the pentagon have, so to speak, been drawn out so that 

 a distinction can be drawn between a central disc and five radiating 

 arms, which are continuations of it. In a Vertebrate, Arthropod, 



450 



