From the polychaetal worms the small-bristle worms originated. The class Small-bristle worms include 4–5 thousand species.
Habitat and meaning
Most small-bristled worms live in the soil (for example, earthworms). Some inhabit fresh and brackish water bodies (for example, the tubifex). Small bristle worms, in particular earthworms, play a huge role in soil formation. They mix the soil, reduce its acidity, and increase fertility. Aquatic small-bristled worms promote self-purification of polluted water bodies and serve as food for fish. Small bristle worms feed on decaying plant debris, but among them there are predators and parasites.
Their body length ranges from 0.5 mm to 3 m. The number of segments in different types of small-bristle worms ranges from 5-7 to 600. All body segments are the same. In sexually mature individuals, a thickening appears in the anterior third of the body – a glandular belt.
They have no parapodia and antennae, and each segment has four pairs of setae – two pairs of dorsal and two pairs of abdominal ones. The bristles are the remains of the supporting elements of the extinct parapodia that their ancestors had. A small number of bristles on the body of these worms gave the name to the entire class – Malochetalovye.
The bristles are so small that they can only be found by touch by sliding your finger from the back of the body of the worm to the front (the figure shows the abdominal bristles at a magnification of 100 (1) and 300 (2) times).
The bristles serve these worms when moving in the soil: bent from front to back, they help the worm to stay in the burrow and quickly move forward.
The glandular cells of the skin epithelium of the worm secrete mucus, which protects the skin from drying out and helps its movement in the soil.
Musculature and movement
The skin and muscle layer, in close contact with each other, form a skin-muscle sac. Between it and the internal organs is a fluid-filled secondary body cavity (whole).
Directly under the skin are the annular muscles, and deeper, the more powerful longitudinal muscles.
With the contraction of the annular muscles, the body of the worm is stretched in length. Contraction of the longitudinal muscles shortens the body. The alternation of these cuts ensures the progression of the worm in the soil.
The digestive system includes the mouth, pharynx, esophagus, middle and hind intestines, and anus.
Moving in the soil, the earthworm swallows its particles, passing them through the intestines, as if eating its own course and assimilating the nutrient particles contained in it.
The ducts of the calcareous glands flow into the esophagus, the substances secreted by these glands neutralize soil acids.
Gas exchange in small-bristled worms is carried out by the entire surface of the body. After heavy rain, when water floods the worm holes and air access to the soil is difficult, earthworms crawl to the soil surface.
The circulatory, nervous, and excretory systems in small-bristle and polychaete worms are similar in structure.
The circulatory system of earthworms is distinguished by the fact that it contains muscular annular vessels capable of contraction – “hearts” located in 7-13 segments.
The nervous system consists of the periopharyngeal nerve ring and the abdominal nerve cord.
Each segment has a nerve node with nerves extending from it.
In connection with the underground way of life, the sense organs of the Small-bristle worms are poorly developed. The organs of touch are sensitive cells located in the skin. There are also cells that receive light.
In contrast to the Polychaete worms, the Lesser Bristles are hermaphrodites.
Their reproductive system is located in several segments of the anterior part of the body. The testes lie in front of the ovaries.
Sexual reproduction takes place with the participation of two individuals. When they touch, they exchange sex cells (the spermatozoa of each of the two worms are transferred into special cavities – the seminal receptacles of the other).
On the front of the body of the worm, swelling is clearly visible – a belt.
The glandular cells of the belts secrete mucus, which, when dry, forms a clutch. Eggs are first laid in it, and then sperm cells come from the seminal receptacles. Fertilization of eggs takes place in a sleeve. After fertilization, the sleeve slides off the body of the worm, thickens and turns into an egg cocoon, in which the eggs develop. At the end of development, small worms emerge from the eggs.
The earthworm has a well-developed ability to restore lost or damaged body parts – that is, such a property as regeneration. If the body of the worm is divided in two, then both parts will be able to exist independently, and the lost organs will recover after a while.