142 ANIMAL ACTIVITIES. 
Compare this starfish with the brittle starfish. 
Summary of Drawings. (a) Sketch of a living 
starfish. 
(4) Sketch of a portion of an ambulacral area show- 
ing the relative positions of the plates and the openings 
for the ambulacral feet. 
(c) Cross-section of an arm to show the relative posi- 
tion of feet, water-tube, plates, and spines. 
(2) Ambulacral feet with ampulle. 
(e) Sketch of a brittle starfish. 
Activities of the Starfish. The starfish belongs to 
the sub-kingdom Echinodermata, animals having hard 
plates in the skin. 
Their movements 
are slow and all 
their activities are of 
a low order, yet they 
are more highly 
specialized than the 
Ccelenterates we 
have just been con- 
sidering. 
Taking Food. 
The mouth of the 
starfish is situated 
on the under side, 
hence this side is 
Fic. 116.—A Starfish. After Agassiz. called the oral side. 
There are no teeth, 
yet the starfish lives on oysters, clams, mussels, and 
other hard-shelled animals. The stomach is an 
elastic bag which fills the central part of the body 
and extends into all the arms or rays, thus making the 
shell simply a protection for this branching, walking 
stomach. This stomach secretes a fluid which par- 
tially paralyzes its prey. If a mussel is too large to 
pass through the mouth, the starfish stretches a part 
of its stomach outside of its body, and enfolds its 
