animal kingdomReading time:
The simplest organisms are single cell animals (protozoa) that include a number of parasite forms (under the flagellate group, only Polymastigines and Diplomonadines include free forms: Sporozoa are all parasites).
In plankton and in the fauna founding activated sludge, there are many protozoa whose significance and whose role are extremely important (see importance of aquatic micro-organisms for the water treatment specialist).
Porifera mark the protozoa transition to metazoa, the primitive sponge comprising a colony of typical protozoa. Then, as we move up to the higher orders of animals, we see important divisions from the viewpoint of planktonic activity, especially in worms, Rotifers (Vermidians) and Arthropods (crustacea and insect larvae).
The classification system put forward in table 5 cannot be regarded as universal; there are variants.