The phylum annelida is made up of a great variety of different species of segmented worms. Indeed there are over 17,000 such annelid species currently known. This includes very familiar examples such as the common earthworm and the leech, as well as many more unfamiliar worms. Besides bodily segmentation, the main defining characteristics of these mostly marine creatures concern the details of their body walls, nervous systems, locomotion, respiration, feeding, and sexual behaviour. This is a very important phylum ecologically, with the earthworm, for example, being important for soil fertility.
The annelid worms are classified as either polychaetes, which make up 12,000 of the 17,000 species, or clitellates, which includes the other 5,000 species. The polychaetes are those that have multiple hairs (chetae) per segment. They also have parapodia for movement and chemosensors on their necks. The clitellates lack these but do have a unique reproductive organ called a clitellum that acts as a store for nourishing their developing eggs.
One of the main characteristics of the phylum annelida is that of a segmented body, with each segment (apart from the head and tail segments) containing the same set of organs, and chetae. They have cuticles that are made up of collagen fibres secreted by the thin epidermis below. The dermis below that is made of connective tissue. Underneath that are muscles around a body cavity known as a coelom that are involved in movement.
The annelids have a brain that appears as a ring around the pharynx that is little more than a couple of pairs of ganglia. Although some species may have more complex structures in place. These may even appear as hindbrain, midbrain, and forebrain. The central nervous system branches out into local pathways in each of the segments. Respiration in the annelids may be through the skin, but could also be through gills. There is great variety in feeding structures across species, such as the use of cilia and sticky pads in the mouth, for example.
Reproduction in the annelids can be sexual or asexual. Polychaetes can use asexual reproduction, in which a new individual buds off from a parent or else the parent divides into two to make two new individuals. But sexual reproduction is also a possibility, with some species changing their strategy depending on circumstances, such as the time of year, for example. The asexual reproductive processes can also be used to help a damaged individual to regenerate.