Page 19 - 新思维科学活动用书7 样章

1 Cells

Hydra has some specialised cells, called sting cells, to help it to catch its food.

These cells contain tiny coiled threads. When a prey animal touches

the trigger on the sting cell, the thread shoots out and wraps around
the prey. Some of these threads may have poisonous
chemicals on them, which kill the prey.

The diagram shows a sting cell before
and after it has been triggered. thread after firing

trigger

coiled thread

cell membrane

cytoplasm

nucleus

1 For each of these parts of Hydra, decide whether it is a cell, a tissue or an organ.

a inner layer of cells

b outer layer of cells

c sting cell

d tentacle

2 In humans, there are several different organs that make up the digestive system.
Does Hydra have a digestive system? Explain your answer.

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