142 
ANIMAL ACTIVITIES. 
Compare this starfish with the brittle starfish. 
Summary of Drawings, (a) Sketch of a living 
starfish. 
(b) 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. 
( d ) Ambulacral feet with ampullae. 
( 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 w e 
have just been con¬ 
sidering. 
Taking Food. 
The mouth of the 
starfish is situated 
on the under side, 
hence this side is 
Fig. ii6.— 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 
