THE STARFISH AND CLOSELY RELATED ANIMALS. 145 
The Sea-urchin. If possible get a living sea-urchin 
and watch its movements in a pail of salt water, or 
better, in a salt-water aquarium. 
What is the shape of the body? 
Do you find oral and aboral surfaces ? 
What is the shape of the spines? How do they 
move ? 
Do you find the ambulacral feet ? 
Examine the mouth and notice the manner of feed- 
ing. 
Hold the living animal in the hand and observe its 
mode of locomotion. 
Place it upside down in the water and watch it. 
Does it have bilateral symmetry or any kind of sym- 
metry ? 
Examine the test or shell which has been freed from 
spines. 
How can you tell the ambulacral from the inter- 
ambulacral plates ? 
How many rows of each kind of plates do you find ? 
What is the shape of these plates ? 
Look at broken pieces of tests. 
Do you find a single plate or tentacle at the end of 
each ambulacral area, as in the starfish (ocular 
plates) ? 
At the ends of the interambulacral areas do you find 
the genital openings ? How many do you find ? 
The small plates, in the circle surrounded by the 
genital plates, correspond to what plates in the starfish ? 
Do you find the madreporic body ? 
What changes would have to occur in the body of 
the starfish to give it the form of the sea-urchin ? 
What parts of the sea-urchin are homologous with 
parts of the starfish ? 
Examine the teeth. How many teeth do you find? 
Write resemblances and differences for starfish and 
sea-urchin. 
Compare a sea-cucumber with a sea-urchin. 
