Hydrozoa,are nothing but a class of invertebrates which belongs to cnidarian phylum.These hydrozoans make up six orders and important characteristics of the most species of hydrozoans is that they show alternation of generations.

In one of theses two generation,the first generation usually consists of the individuals which is popularly called as the polyps and important feature of this polyps is that it is almost always colonial.The reason why the colony is called as hydrozoan is becauuse of the resemblance of its individual polyps to the noncolonial hydra.

When we consider the other other generation whose members are known as medusae,usually consists of free-swimming and the sexual individuals which produce either eggs or sperm., e.g., the familiar jellyfish belongs to typical medusa.

Here every fertilized egg will develops into a ciliated larva which is called as planula,and from this the hydroid develops and by budding this hydroid will gives rise to medusae.

Usually the Hydrozoans differ in their conformity to the typical alternation of generations and the order to which the hydra belongs has small medusae and sometimes no free-swimming forms at all.

The siphonophoran order,which contains the Portuguese man-of-war, normally have a short-lived medusa stage,and here the larvae develops into medusalike organisms and from this the colony arises and they have extremely specialized individual members in siphonophoran colony.

Normally the polyps have a specialized defense mechanism as they produce a venom which is strong enough even to kill a human being.

Apart from this another order has a short-lived polyp stage and in some of the species there is no polyp stage.
Its members mostly spend their lives as free-swimming medusae.

Then the fourth order is usually the combination traits of polyps and medusae,which normally resembles free-swimming and ciliated hydras.

Naturally the two primarily tropical orders usually characterized by an important feature which secrete a large, hard, calcareous exoskeleton and also often form corals.

When it comes to the scientific classification,

#)The class Hydrozoa belongs to the phylum Cnidaria

#)Hydra belong to the order Hydroida

#)siphonophores to the order Siphonophora

#)Species with a short-lived or nonexistent polyp stage usually belong to the order Trachylina.

#)The Species which resembles free-swimming polyps normally belongs to the order Actinulida.

#)Stylasterina and Milleporina are the two primarily tropical orders are and sometimes Stylasterina is also considered as suborder.