Errant vs. Sedentary
The worm that lives an errant lifestyle would be best illustrated by the Nereis, a type of annelida able to actively move around its environment to feed.
The head of a Neresis has two eyes and several sensory appendages.
The remaining 100+ segments in an errant body structure are all similar, each with a pair of distinct appendages known as parapodia, which are highly muscular lobes covered in setae, bristles used for crawling and swimming.
Organisms with errang body structures tend to be carnivores. It feeds on small live organisms or fragments of dead ones, using powerful jaws to bite chunks off of its food, and digests it through a straight, tubular gut. Any indigestible material is excreted through the anus at the opposite end.
The way the Neresis increases in length is by growing segments that extend from its posterior.
Most worms with sedentary lifestyles tend not to move around too much in search for food, either rooting themselves in one area or being bottom-feeding scavengers, as illustrated by the feather duster worm.
Both errant and sedentary lifestyles share bodies built according to a "segmental plan", in which certain structures that carry out specific body funtions are repeated in each segment. They also conduct gas exchange (respiration- exchange of oxygen and carbon dioxide) through the entirety of their skin, which is why earthworms come out by the thousands whenever it rains; they will otherwise drown in their tunnels.
Most annelida also tend to be hermaphrodites, with a single complete set of reproductive hormones, doubling the chances of finding a mate in time when it is the right season to breed.
There are several body systems that travel through all the segments of the worm.
- A well developed circulatory system
- A separated digestive system
- A complex nervous system
