We can classify annelids using as criteria the presence or absence of hair-like structures and the amount of these bristles.
There are three groups of annelids: oligoquettes, polychaetes and aquetos.
By the meaning of these words, it is possible to identify how the bristles (ketos) of these animals are: oligo means "few"; poly means "many"; and The means "without".
They have few bristles per ring. There are no parapodes (small body projections that aid locomotion), and no differentiated head from the rest of the body.
The main representative of this group is worm. She has skin covered by a thin film and produces a slimy substance; This mucus reduces friction with the soil, protects the skin from contact with possible toxic substances and maintains moisture, which is critical for skin breathing.
In this animal, the clitelo - a lighter ring where animals unite in cross-fertilization, exchanging sperm. After breeding, each of the worms releases into the soil a cocoon full of eggs. A few days later, these young worms hatch.
- The digestive system is formed by a mouth; one chatwhich looks like a great camera; a gizzard through which food is crushed; a long gut, which ends in anus, situated in the last ring of the body.
- O circulatory system is closed, and in it the blood circulates within the vessels. The blood has hemoglobin, the same red pigment that we humans have.
- The nervous system is formed by nerve cells that coordinate various body functions.
The earthworm plays an important role in soil fertility. It digs "tunnels", acts as a plow, increasing aeration and water circulation. In addition, their feces contain nourishing substances that mix with the soil and act as fertilizer, fertilizing the soil.
They have many bristles in each segment, ie in each ring. Each ring has a pair of lateral projections, the parapodes, on which the bristles are implanted.
The polychaetes are carnivores. They are often cannibals, that is, they devour other polychaetes.
Aquetos (also called hirudinous) have no bristles and suction cups, which aid in fixation and locomotion.
In this group is the leech. She is hermaphrodite and lives in moist, marshy soil or in fresh water. There are also some marine species.
The leech sucks the blood of other animals through the suction cups, but can also feed on earthworms and animal remains. It is small, its length varies from 1 to 20 centimeters.