SECTION VII. PHYLUM ECHINODERMATA 



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 

 (p. 129); 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 Star-fish and Sea-urchin as the organs of 

 locomotion, by means of which the animal creeps slowly 

 along. Examination of the internal 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 

 Star-fish, Asterias rubens, is enclosed in a tough, hard 

 integument, containing numerous plates, or ossicles as they are 

 termed, of calcareous material. This exoskeleton is not 



