CHAPTER XVIII. 

 PHYLUM IX: ECHINODERMATA. 



223. General Survey. This branch of animals is found 

 only in the sea. It is perhaps the easiest of the lower 

 groups for the beginner to recognize. While its members 

 differ a great deal in general form, they usually have 5 

 (sometimes more) rays, or rows of structures, running 

 radially from the mouth outward. In the simplest types 

 these radial parts are in definite arms, as in the common 

 starfish (Fig. 62). Usually the body is covered with 

 spines which are attached to- bony plates in the body wall 

 (hence the name echinoderm) . Common names of the 

 classes in the branch are : starfishes, sea-urchins, brittle- 

 stars, sea-lilies, sea-cucumbers. 



In addition to the radial symmetry and the spiny skin, 

 the following facts make the group interesting to the 

 student : There is no head properly speaking, the mouth 

 being located in the middle point from which the rays 

 diverge; all the members of the group are sluggish and 

 very poor movers, in the adult ; the young are bilaterally 

 symmetrical and have to pass through a metamorphosis 

 before they become radiate. 



224. Starfish: a Practical Exercise for the Laboratory. 



Dry specimens will serve for this study, but one or more 



alcoholic specimens will be valuable. 



General Form. Indicate the signs of radial symmetry. 



Is there a central disc? If so, how large is it in relation 



to the arms? Along how many lines can you divide the 



206 



