HYDRO IDS 
165 
Figure 153. — A 
Hydroid Colony. 
It resembles a 
plant and is often 
mistaken for one. 
Figure 154.— The Medusa Known as 
Pelagia. 
It is found swimming on the surface of 
the water far from land. 
medusae are set free from the hydroids, they swim about and 
capture their own food. Each medusa is provided with either 
ovaries (o'va-riz), organs which 
produce egg cells, or sperma- 
ries (sper'ma-riz), organs which 
produce sperm cells. When the 
eggs and sperms mature, they 
are discharged into the water. 
A single sperm cell must fuse 
with an egg cell before the egg 
can begin to grow. The union 
of these two cells is called 
fertilization. The egg grows 
into an embryo (em'bn-o), an 
immature stage varying with 
different animals, and this 
gradually changes into a small 
hydroid. The several steps 
When the medusa buds shown 
forming at A in Figure 152 mature, 
they are set free in the water and 
look like this photomicrograph of a 
free swimming medusa (greatly 
enlarged). 
