Hydrozoa (hydrozoans, from ancient Greek ὕδρα, hydra, "sea serpent" and ζῷον, zoon, "animal") are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most living in salt water. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony. A few genera within this class live in fresh water. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria.
Some examples of hydrozoans are the freshwater jelly (Craspedacusta sowerbyi), freshwater polyps (Hydra), Obelia, Portuguese man o' war (Physalia physalis), chondrophores (Porpitidae), "air fern" (Sertularia argentea), and pink-hearted hydroids (Tubularia).
Most hydrozoan species include both a polypoid and a medusoid stage in their lifecycles, although a number of them have only one or the other. For example, Hydra has no medusoid stage, while Liriope lacks the hydroid stage.
The hydroid form is usually colonial, with multiple polyps connected by tubelike hydrocauli. The hollow cavity in the middle of the polyp extends into the associated hydrocaulus, so that all the individuals of the colony are intimately connected. Where the hydrocaulus runs along the substrate, it form a horizontal root-like stolon that anchors the colony to the bottom.
A great bear in the woods
He wants all he sees
From the snakes in the grass
To the chimpanzees
We all know it ain't fair
Well, it's a jungle out there
So go, bang your drum
Mr. Big Baboon
What monkey see
Monkey will do
Boom, boom, boom, boom
Mr. Big Baboon
Mad dog on the prowl
Sees the bear in the trees
They can never be friends
They're natural enemies
So you better beware
Well, it's a jungle out there
So go, bang your drum
Mr. Big Baboon
What monkey see
Monkey will do
Boom, boom, boom, boom
Mr. Big Baboon
Mr. Big Baboon
Sees the dog and the bear
It's a three way tie
It's a stalemate there
Facin' off eye to eye
Well, it's a jungle out there
So go, bang your drum
Mr. Big Baboon
What monkey see
Monkey will do
Boom, boom, boom, boom
Mr. Big Baboon
So go, bang your drum
Mr. Big Baboon
You can all join in
Monkey see monkey do
Boom, boom, boom, boom