Contrary to the previously discussed phyla, annelids (annelus = small ring), have the body divided into segments or essentially ring-like metamers of one another.
These animals are quite old on Earth, with fossils from this pre-Cambric period, though the first undoubtedly segmented worms are from the Medium Cambric. They are thought to have evolved from a platelminte type ancestor.
The segmented body is externally visible in the form of rings, with well-marked grooves separated from each other. Some structures, such as the excretory organs and ganglia of the nervous system, repeat internally in each segment, also called metamer.
This is why the body of the annelids is metamerized. Internally, metamers are separated from each other by dividing walls known as septa.
The compartmentalization of the cheloma increases the accuracy of the movements as pressure can be applied to different areas of the liquid. Waves of muscular contraction, controlled by the nervous system, pass through the body, alternately stretching and contracting groups of segments, forcing the animal's body to move.
The displacement of annelids is further aided by the presence of bristles, in the ventral part of the animals (except in leeches). These chitinous bristles prevent the animal from sliding backwards, reinforcing the forward movement of the circular and longitudinal muscle layers.
In earthworms there are 4 rows of bristles, while in polychaetes there are many more, applied to body wall expansions called parapodes, located laterally in each segment and considered sketches of locomotor organs.
Metamerization has the added advantage of allowing segment specialization or segment groups for different functions, although this aspect has not been much developed in annelids (unlike arthropods).
Terrestrial annelids have an epidermis with sensory cells, covered by a thin and transparent cuticle, which protects them from desiccation. There are also mucous glands that help keep the surface moist, essential for skin breathing. For this reason the epidermis is very vascularized.
Annelids have segmented excretory system, with metanephrid pairs in each segment. Nephrids are thin, coiled tubules (in contact with blood vessels from which they excrete), with an open ciliary funnel at one end - nephrostome - which opens in the celomic liquid (from which they also excrete) from each segment. The other end - nephridopore or excretory pore - opens on the body surface.
The nervous system has in the anterior part a pair of cerebral ganglia connected to a circumfarinic ring and ganglia in each segment, which attach to a double and massive ventral nerve cord. Taken together, the nervous system resembles a rope ladder.
O digestive system is complete and has different specialized regions, namely:
- sucking pharynx;
- gizzard - crushes the food, acting like the teeth, performing a mechanical digestion;
- intestine - where digestion, extracellular and chemical is performed. In the gut there is typically a dorsal fold called tiflosole, which allows for an increase in the area of nutrient absorption.
Most annelids feed on decaying particles, microorganisms and larvae.
O circulatory system It is closed and complex, with longitudinal dorsal vessels, where blood circulates towards the anterior part, and ventral, where blood circulates to the posterior region, connected by transverse vessels in each segment.
In the anterior region, some (four or five, depending on the species) of these lateral vessels are surrounded by muscle cells, functioning as lateral hearts or aortic arches.