172 
THE STARFISH FAMILY 
of a clam or oyster which is protected by hard shells. It 
is now known that the starfish attaches its tube feet to each 
side of the clam and by constantly pulling tires out the 
strong muscles that hold the shell together. As soon as 
these muscles relax, the 
shell opens. The stomach 
is then pushed out, en¬ 
veloping the clam. The 
digestive fluid contain¬ 
ing enzymes is secreted 
and the dissolved clam is 
absorbed as food. 
153. Respiration.— 
Oxygen is taken from the 
water and carbon dioxide 
given off through little 
thin-walled, gill-like pro¬ 
cesses which cover the 
upper surface of the disk 
and arms. These gill-like processes project through holes 
in the exoskeleton. 
154. Excretion. — There are no special organs in the star¬ 
fish for the removal of wastes. 
155. Nervous System. — The starfish possesses a definite 
arrangement of nerves. In this respect, it shows a marked 
advance over the hydra. There are five nerves, one in each 
arm, all of which join a nerve collar that surrounds the mouth. 
There is no brain. This nervous system of five nerves and 
a nerve ring receives stimuli and controls the slowly contract¬ 
ing muscles. 
156. Life History of the Starfish. — In the starfish the 
sexes are distinct. The female discharges eggs into the 
water and the male discharges sperm cells into the water. 
The sperms being constantly in motion, are likely to meet 
the eggs. When this occurs, a sperm cell enters an egg, 
