THE STARFISH AND CLOSELY RELATED ANIMALS. 141 
The walking, or ambulacral, surface is called the 
ambulacral area. 
How many rows of ambulacral plates in each ray? 
How many rows in all ? 
Do the ambulacral feet pass through these plates or 
between them? 
The regular plates situated on either side of the 
ambulacral areas are called cxterambulacral plates, and 
the surfaces they cover are called interambulacral 
areas. How many rows of interambulacral plates do 
you find in each ray ? How many rows in all ? 
How do the spines on these plates differ from those 
on the irregular plates of the aboral surface ? 
The plates on the aboral surface may be called 
body-plates. Can you find the single plate with its 
ambulacral foot at the end of each ray ? 
With a partly dissected specimen, which has been 
prepared by hardening in alcohol, laying open the 
upper or aboral surface of one of the rays and removing 
the digestive organs, notice the ampulle or internal 
enlargements of the ambulacral feet. What is the use 
of the ampullze in locomotion ? 
These ampullze are connected with a tube running 
along the ray in the angle made by the two rows of 
ambulacral plates. Trace this tube, which supplies the 
ambulacral feet with water, up to the tube running 
around the mouth. Find the stone canal connecting 
this tube with the madreporic body. What do you 
think may be a use of the madreporic body ? 
Examine portions of the skeleton of a starfish which 
have been decalcified by allowing them to soak in 
about ten per cent. nitric acid for a few days. Compare 
these with similar portions which have been treated 
with weak caustic potash until the fleshy matter has 
been removed. Are the plates formed inside of the 
fleshy skin or outside of it? 
Write resemblances and differences for starfish and 
hydra. 
