Many single-celled animals live free in the water but others have a stalk and attach themselves to substrates. The ciliated protozoa that are classified as suctoria are specialised, stalked ciliates. The basic shape is an inverted bell for a body that is lined with spikey tentacles on the top and hasa stalk on the narrow end. The bell is transparent and the internal structure of the organism can be seen using a microscope. One can also see that there are two nuclei, one large and one small, in common with other ciliates. The basic classification of suctorial ciliates is: phylum Protozoa, class Ciliophora, order Suctoria. This may change as protozoan classification is in a rapidly evolving state, with new information constantly altering our understanding of these groups.
Suctoria can reproduce by binary fission but commonly reproduce by budding. The buds become the ciliated larvae and may form in a brood chamber or on the surface of the parent, before swimming free. The suctoria have cilia in thesefree-swimming larval stages, but once they develop and settle, the cilia are replaced by spike-like structures with tubes down the middle. As an unsuspecting prey swims by, the suctorial ciliate stabs it with a sharp tube and sucks its protoplasm out. Their prey are usually free-swimming ciliates, flagellates or bacteria. Some suctoria live alone while others live in colonies. I have seen them attached in their thousands to rotting plant matter in ponds, where bacterial prey are plentiful.
Suctoria are common in freshwater environments, while others live in the ocean. Marine environments, because of the depth of the water, can be short of suitable attachment sites for these animals.So many of them have become epibionts, which means that they attach themselves to other animals. Crustaceans are good hosts because of their hard exoskeletons. They have been found attached to copepods and amphipods, among other crustacea. The suctoria are commensals and do not harm their hosts. They just need a solid structure on which to attach their stalks.
There is still much to be learned about these strange little creatures. Their classification alone needs to be worked out. Their life styles and ecology are still largely unstudied. The fossil record is poor for most single celled animals but they have probably been around for hundreds of millions of years. When they are present in a pond, they are an indication of good water quality and stable conditions as it takes time for colonies to build up and they need good clean oxygenated water to survive and thrive.
If you have access to a dissecting microscope, have a look at the undersides of floating vegetation. It is quite likely that, with a little patience, you will find some of these stalked creatures and you might even see them capturing their prey.