CHAPTER V 



BRANCH ECHINODERMATA : THE STARFISHES AND SEA- 

 URCHINS 



THE STARFISH 



Starfishes furnish excellent objects for study, whether 

 freshly killed or preserved in alcohol or formalin. The 

 internal structure is best made out by dissecting under 

 water. Of course the larger the specimens are the more 

 satisfactory will be the results. Dried specimens are 

 desirable for the study of the outer surface. 



EXTERNAL ANATOMY 



I. — What gives the animal its star-like appearance .■■ Dis- 



tinguish the flat ventral surface from the more rounded 

 dorsal surface. To understand that the body is pentag- 

 onal-shaped, imagine the rays cut off at their bases. 



II. — Compare the animal's symmetry with that of the frog. 

 Notice the apparent absence of anterior and posterior 

 ends. Such an arrangement may be called radial sym- 

 metry. The following method, however, will enable 

 you to understand that the arrangement of parts ap- 

 proaches a bilateral symmetry, or such an arrangement 

 of parts that the whole may be divided into two similar 

 halves : Find a smooth circular white, yellow, or orange 

 spot — the madreporite — near one side of the dorsal 

 disk and near the angle formed by two adjacent rays. 

 Will an imaginary line drawn from the madreporite 

 across the starfish divide it into two similar parts ? 



3S 



