44 Laboratory Guide in Zoology 



THE CLASSIFICATION OF THE ECHINODERMATA 



The starfish is perhaps the most typical representative 

 of the branch Echinodermata, which includes also the sea- 

 urchins, the sea-slugs or sea-cucumbers, the sea-lilies or 

 feather-stars and the brittle-stars. In the members of this 

 phylum the arrangement of the parts in a circle around a 

 common centre is the most striking general character. 

 This is obvious in the starfish, and may easily be seen in 

 the other members of the group. There is also a calcare- 

 ous outer skeleton which varies much in its texture, but 

 the frequency with which it is furnished with spiny pro- 

 jections led to the name Echinodermata. All members of 

 the phylum live only in salt water. There are two open- 

 ings to the alimentary canal. There is also a distinct 

 nervous system, a circulatory system, a water system for 

 locomotion, and other complicated organs. 



There are five principal classes of the Echinodermata, 

 namely : — 



I. — Asteroidea. The starfishes are the typical representa- 



tives of this class. They abound in almost every sea, 

 and may be found partly buried in the sand or con- 

 cealed beneath the shelter of the rocks. As the name 

 implies, they are star-shaped, generally with five rays. 

 In color they vary considerably, but they are com- 

 monly some shade of yellow, orange, or red. 



II. — Ophiuroidea. This class is made up of the peculiar 

 sand-stars and brittle-stars, which have the arms longer 

 and more distinctly separated from the central portion 

 of the body than do the starfishes. 



III. — Echinoidea. The spiny sea-urchins form this group. 

 In these the arms are absent, the body being rounded, 



