SECTION VII.— PHYLUM ECHINODER- 

 MATA 



The starfishes, brittle-stars, sea-urchins, feather-stars, and 

 their allies, many of which are familiar objects on the sea- 

 shore, are grouped together as the phylum Echinodermata. 

 Even a superficial comparison of a starfish, a brittle-star, and 

 a sea-urchin will reveal unmistakable points of agreement. 

 All have a hard surface more or less abundantly provided 

 with pointed spines : in all the symmetry is distinctly 

 radial ; and, if the animals are examined in the living 

 condition while immersed in sea-water, it will be found that 

 all are provided with rows of soft retractile tubular append- 

 ages acting in the starfish and sea-urchin as the organs of 

 locomotion by means of which the animal creeps slowly 

 along. Examination of the external structure shows, as will 

 presently become evident, that the resemblance is not a 

 merely superficial one, but extends to all the systems of 

 internal organs. 



1. THE ASTEROIDEA 



The body of a starfish, such as the common English red 

 starfish, Asterias rubens, or A. vulgaris, of our American 

 coast, is enclosed in a tough, hard integument, containing 

 numerous plates or ossicles, as they are termed, of calcareous 

 material. This exoskeleton is not completely rigid in the 



'57 



