CHAPTER XVIII 

 THE STARFISH AND SOME ALLIES: ECHINODERMA 



Let the mere star-fish in his vault 



Crawl in a wash of weed, indeed, 

 Rose-jacynth to the finger-tips. 



ROBERT BROWNING. 



THE STARFISH 



Habitat and Distribution. The purple starfish (Aste'rias 

 vulya'ris, Fig. 118) is found most abundantly north of Cape 

 Cod in tide-pools on rocky shores, and in deep water near 

 the shore. It is only in the warmer season, however, that one 

 may witness a scene like that portrayed in the illustration. 

 When the winter storms come on, starfishes and 'many other 

 tide-pool animals that are not fixed permanently migrate to 

 the deeper water, in order to be in more protected places. 



External Structure. Up to this chapter we have been giv- 

 ing our attention to animals that have a perfect, or slightly 

 modified, bilateral symmetry. The starfish evidently has a 

 plan of structure for which we must find some other name. 

 In a specimen we observe five arms extending from a central 

 region. The position of the arms with reference to the cen- 

 tral region or disk is practically the same as the position of 

 radii in a circle. Hence we say that the starfish is radially 

 symmetrical. We find it necessary also to change a few 

 terms of location. In the picture of the tide-pool study we 

 are looking down upon the aboral surface of the three star- 

 fishes. The opposite side has the mouth at the center, and 

 for that reason is called the oral surface. 



The aboral surface and part of the oral surface of Asterias 

 is thickly set with calcareous spines that arise from small 



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