22 
EANHS Bulletin 30(1) 
feed mainly on jellyfish. Sea slugs, 
which are marine molluscs, cat sea 
anemones. The anemones' stinging 
capsules remain active, and 
become part of the sea slug's own 
defences. 
Cnidarians may have algae in their 
tissues. Algae arc living things like 
plants: In sunlight, they use water, 
minerals and carbon dioxide to make 
food. Some algae arc large, but the 
ones living in corals and other 
cnidarians arc very small One alga 
may be just one single cell. Most coral 
polyps have single-celled algae in their 
bodies. In clear, shallow tropical seas, 
there is enough sunlight for algae to 
do their work of making food The 
coral polyp eats food made by the 
algae, and the algae are protected by 
being inside the coral. Single-celled 
algae give some living corals and sea 
anemones a brilliant colour. There 
may be up to 30,000 single-celled 
algae in one cubic millimetre of 
polyp! The algae also help coral 
polyps to produce calcium carbonate 
to build ihcir stony walls. 
Cnidarians may grow big. Some 
of the jellyfish that live in cold waters 
may be three metres across. Their 
tentacles are more than 30 metres long 
They are among the largest of all 
invertebrates. 
Cnidarians may live long. One sea 
anemone was kept in an aquarium in 
California. USA. for 70 years A fresh- 
water hydra, however, lives for just 
one season. 
Cnidarians live as individuals or 
groups. A coral polyp usually builds 
a separate stony cup around itself. The 
‘mushroom coral’ is a large polyp that 
lives alone in its mushroom-shaped 
tup. Other coral polyps build their 
stony walls dose together, forming a 
colony The whole colony has a 
particular shape, such as plate coral 
In a brain coral colony, there are 
grooves on the coral mass, instead ot 
individual cups The bodies of several 
coral polyps are together in the same 
groove. Some jellyfish are really many 
animals grouped together into what 
looks like one jellyfish. 
Cnidarians reproduce in two 
ways. A polyp may form a bud which 
breaks off as a new polyp This is 
called asexual reproduction. A coral 
colony grows by budding Coral polyps 
also shed sperm into the sea. Some 
sperms land on polyps containing eggs, 
and fertilise the eggs. Each fertilised 
egg becomes a free-floating larva. The 
larva later lands on a surface, becomes 
fixed, and begins to grow into a coral. 
Some cnidarians have several life 
stages. One stage is a swimming 
animal like a jellyfish. This stage, 
called the medusa, sheds eggs and 
sperms into the sea. An egg is fertilised 
by a sperm, and becomes a little larva. 
The larva lands on a solid surface and 
grows into a polyp. The polyp may bud 
into other polyps and form a colony. 
Eventually, little medusae bud off from 
the polyp or colony. They break off. 
swim away, and the cycle begins 
again. Corals do not have a medusa 
