There are many different sorts of worms but the height of worm evolution is found in the annelids or segmented worms. There are four classes in this phylum. Oligochaetes are terrestrial and freshwater worms that have few hairs. The most commonly recognised are the earthworms. Polychaetes are marine worms with many hairs. There are numerous species of this successful group. The Hirudinea are the leeches, which are normally found in freshwater and which have adapted for the most part to a blood-sucking, ectoparasitic lifestyle. The last and least known class is the Branchiobdellida. These are highly modified worms that live as commensals on crayfish. They have no setae or hairs on their bodies and they have one posterior but no anterior sucker.
It appears that annelids arose from flatworms and are related to the molluscs. It is probable that arthropods arose from annelid stock since they have kept the segmentation characteristic of this group. Within the annelid phylum, the polychaetes are considered to be the most primitive. Originally some of these were designated the archiannelida and thought to be the most primitve but now they are just considered to be degenerate. Oligochaetes arose from burrowing polychaetes although it is not clear from which exact group, while leeches and branchiobdellids probably arose separately from oligochaete stock that altered to become ectoparasites and commensals.
The Polychaetes are the most diverse class of annelids, with about 4000 species being found in every marine habitat and occupying a number of niches. There are free-living, swimming polychaetes and there are burrowers that form an important part of the benthos. Many burrowing polychaetes spend their larval days in the plankton before settling. Polychaetes are divided into three subclasses: the Errantia, which do not have the body divided into distinct regions; the Sedentaria that have the body divided into specialised regions; and the archiannelida that are reduced with no parapodia and few segments. Originally the names of the first two groups were supposed to indicate lifestyle, however within the Errantia there are crawling, swimming and burrowing forms as well as a few tube dwellers. They are equipped with strong jaws and most are predatory, although some are mud eaters. The sedentaria usually live in tubes and have no jaws. They are normally detritus feeders and use tentacles to gather their food. Some have only two tentacles while others have more. Some are simple while others are quite complex in structure.
Oligochaetes are far less diverse than the polychaetes in body form. There are about 3000 species and they mostly live in the soil but a few are found in marine and freshwater environments as well. They have no parapodia and few setae. The head is reduced and has no sense organs. The terrestrial earthworms are larger than the aquatic species and the largest is the Australian species, Megascolides australis, which can be three meters long. All terrestrial earthworms are herbivorous detrital eaters and are important decomposers and soil makers. Earthworms move vast quantities of soil each year, aerating it and digesting it and inoculating it with bacteria. They are vital to healthy soils.
Aquatic oligochaetes have gills. They consume detritus and algae by extruding the mucous-covered pharynx, and then swallowing the particles that stick to it when it is retracted. They also contribute to the circulation of bottom sediments.
Not all leeches are blood suckers but all have the characteristic suckers seen on the bloodsucking varieties. Leeches spend most of their lives in fresh water environments, only attaching to a host occasionally for a blood meal. Blood is a rich food source, high in protein and this allows leeches to live for long periods between feeds.
Meglitsch (p 364) had this to say about the annelids: “Nearly everyone knows some of the marine polychaetes, the earthworms, or the leeches and so has some familiarity with annelids. … Annelids are interesting for a variety of reasons. No other animals so clearly illustrate the principles of metameric organisation and they are much studied in introductory [biology] courses for this reason. Comparative studies of annelids have been extremely important in discussions of phylogeny. They make good physiological subjects and they are in a very interesting stage of complexity for the study of some basic functions. They include some parasites with interesting peculiarities, and some blood-suckers that are involved in disease transmission. It is evident that Annelida is one of the major animal phyla.”
Reference: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press.