the hylum is divided into 3 classes
- the hydrozoa
- the scyphozoa
- the actinozoa
A- class hydrozoa:
some of the letter are reef bulding corals show an altenation of asexual( hydroid) and sexual (medusoid).
1- Hydra:the entron os the only cavity to the body.
2- obelia: it is dimorphic, exhibiting two forms in life cycle, a hydroid and mdusoid.
a- the colony
b- the medusa: the general form, umberella like.
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