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Basic Anatomy and Physiology of Annelids
The biological structure of the Annelida (segmented worms) is centered around a high level of organ specialisation found in these coelomates (animals with a fluid-filled cavity).
This segment explains how the internal and external anatomical features of marine bristle worms work, with extra details about the special adaptations that help them crawl around the ocean floor.
What's the Anatomy of a Polychaete Worm?
Marine annelids (Class Polychaeta) are segmented, coelomate worms with repeating units (a condition called 'metamerism').
They have a 'closed circulatory system', meaning their blood is always contained with a network of vessels.
Having a complete digestive tract also means they take in food through the mouth and excrete waste products through the anus.
Still, one of the standout features of marine annelids is a complex level of special organs compared with simpler worms, such as the flatworms (Platyhelminthes) and roundworms (parasites).
Pro Tip: The main section contains more information about the annelida species, including what they eat, and how these soft-bodied marine worms reproduce.
Segmented Body Organisation
A distinctive characteristic of Annelida is the external segmentation that gives the appearance of external 'ringlike' annulations.
The division of the body in this formation of repeating segments (called metameres) helps to facilitate movement.
Moving on...
Annelids are true coelomate worms, so they have a fluid-filled body cavity. In essence, it is a hydrostatic skeleton that forms a separation between the digestive tract and the outer body wall (cuticle).
So, what is a cuticle in annelids? In fact, it is a structure composed of a flexible collagenous (protein-based) layer that sits over the epidermis and contains longitudinal and circular muscles.
Annelid Exterior Features
The anterior end (front head) contains the prostomium, a shelf-like structure or lobe that is often the location for sensory organs. The main body section (called the peristomium) is the part that surrounds the mouth.
Annelid setae (sometimes called chaetae) are tiny chitinous, hair-like bristles that cover the body in varying sizes and quantities - depending on the species.
Here's the thing:
As with most bristle worms, setae are used for traction and locomotion - even though they are absent in leeches.
The pygidium may be non-segmented and it's located at the opposite end to the mouth. Hence, the posterior end (tail) contains the part that removes waste from the body - the annelid's anus.
Annelid Digestive System
The digestive tract facilitates a one-way process that begins in the mouth.
Food enters the pharynx and esophagus, before it travels into the food storage organ - called a 'crop'.
Most polychaete worms eat small bivalves and crustaceans.
As a consequence of that, the gizzard is the place where the grinding takes place before the remains eventually end up in the intestine and then the anus.
Circulation and Respiration
Unlike many saltwater invertebrates, having a closed circulation means the dorsal and ventral blood vessels are joined by lateral vessels. The plasma contains hemoglobin so the colour of annelid blood is red.
For most species, respiration occurs through the skin (known as cutaneous respiration). Whereas others may use parapodia with gill-like structures for gas exchange, such as bristle worms and feather duster worms (Sabellidae).
So now you must be asking... does annelid have a heart? Yes they do! However, the muscular, heart-like structures vary in size and numbers between different types of annelids.
Some have a number of tiny hearts, whereas others may only have one, and some have no 'definitive' heart at all.
Annelid Nervous System
The cerebral ganglia (annelid's brain) is slightly above the muscular, suction-producing organ (the pharynx). But, the central nerve cord runs on the underside of the body.
Segmental ganglia are capable of controlling specific movements, and their sensory structures are light-sensitive. They also have touch receptors and some species have eyes (often with pigment cups and lenses).
Metanephridia in Annelids
The primary functions of the metanephridia (the excretory organs) include filtering body fluid, removing metabolic waste, and maintaining a healthy balance of salt and water.
Pro Tip: If you're wondering "how do annelids reproduce", head over to the main section for further information about their reproductive strategies.