These are small organisms that drift in large water bodies like seas and oceans. They have eight major groups as briefly described below: Copepods : Their habitat includes both marine and freshwater but mostly fresh water. Their bodies are cylindrical with a small rounded head that has a large pair of antennae which enables them to swim.
Their small size means that their respiratory system is not highly developed. They therefore lack a heart or blood vessels. Cnidarians: They have stinging tentacles for predation and mostly found near the surfaces of large salt water bodie like oceans. An example is a jelly fish. Chaetognatha: They have fins on their body sides that enables to them to move in the water. On their heads are curved spines that help them catch prey.
They are also transparent and are commonly called arrow worms.Cladocerans: The thorax and abdomen is unsegmented while the head faces downward and has two antennae. The second one is used for swimming. During the day they stay under water but at night they move to the water surface. They mostly live in fresh water habitats. Rostifera: They aree microscopic and live mainly in fresh water.
An example is ptyugura pilula Amphipoda: Their bodies are divided into three segments; head, thorax and abdomen. The head has an antenna and a pair of compound eyes. They include Mollusca:They live in both marine and freshwater habitats. They are invertebrates that have a hard exoskeleton on their bodies. An example is an octopus.
Protistsa: They have a nucleus and a majority are unicellular with the remaining small num ber being multi cellular. Examples are the protozoa and algae.