Internal Anatomy

The muscular, digestive, vascular and nervous systems consist of serially repeating subunits across the length of the worm’s body which unite all of the segments of the worm. Internal organs such as the excretory nephridia are also repeated throughout the segments.

Musculature

The muscular fibres form a circular tissue beneath the epidermis of the polychaete. Nereis spp. which have well-developed parapodia also have think longitudinal muscles which are located beneath the circular muscular layer.

Sensory system

Sensory structures including photreceptors, chemoreceptors and mechanoreceptors are typically unicellular structures. They are distributed all over the body but are more concentrated on the head and on appendages. The most important sensory organs include the following:

• nuchal organs – unique to polychates, chemoreceptive organs which are used to detect food.
• ocelli (eyes) – well-developed in Nereis, two pairs, generally pigment cups (probably only able to detect the intensity of light and its direction)
• statocysts – orientating function

Some of the most important sensory appendages occur on the head region of the polychaete including the antennae, palps and tentacular cirri. All appendages are thought to contain cells for mechanosensory and chemosensory purposes (Ruppert et al., 2004). Photoreceptors and mechanoreceptors in Nereis spp. cause the polychaete to respond rapidly to sudden stimuli, this may include a change in the intensity of light or a mechanical stimulus such as an unexpected touch of the body (Evans, 1963). With repeated stimulation the worms have been proven to become habituated and will no longer respond (Clark, 1960).

Circulatory system

The circulatory system of the nereids consist of a dorsal and ventral blood vessel as well as branching vessels which supply the coelom and body tissues with the necessary nutrients; small vessels connect the dorsal and ventral blood vessels (Beesley et al., 2000). The vessels like those of most other marine invertebrates are comprised of extra-cellular matrix rather than endothelial cells. The hemal fluid is able to hold large concentrations of oxygen allowing nereids to survive in anaerobic intertidal habitats (Beesley et al., 2000).

Dorsal blood vessel

Excretory system

Excretion of excess nutrients and waste material occurs through structures called nephridia, or "little kidneys". In species of Nereis nephridia are present in all segments except the most anterior and posterior segments. Nephridia also play an important role in osmoregulation.

Nervous system

The nervous system of polychaete worms consists of a pair of ventral, longitudinal nerve cords and various ganglia or bundles of nerve tissues. In species of Nereis the two nerve cords are fused along the midline of the worm. Each segment bears pair of ganglia which a joined to each other and to the longitudinal nerve cords. This results in a ladder-like structure of the nervous system. Further ganglia are present at the base of the parapodia and are known as pedal ganglia. These additional ganglia are likely to function primarily in the complex movements of the trunk and paradodia (Ruppert et al., 2004).

Unlike many of the other polychaete families, members of the family Nereididae lack a central, integrated brain (Beesley et al., 2000).

Reproductive system

Sexual reproduction

Polychaetes generally reproduce via sexual means whereby gametes are either transferred to an individual or a spawning event occures. Hermaphroditism is unlikely to occur in marine nereids. Gonads may be present in specialised segments or in all segments of the worm depending on the taxa. Gonads open in the coemic cavity whereby they are excreted.

Species of nereid polychaete are known to undergo epitoky, however, there are no records of such morphological modifications occurring in Australia (Beesley et al., 2000). Modifications include: 

• Enlarged eyes
• Parapodia with flattened, paddle-like lobes
• Flattened compound chaetae
• Some degree of regionalisation of body segments
• Sexual dimorphism

Freshwater species from the same family are known to complete their life cycle in 1-1.5 years. The life cycle length of marine species is currently unknown (Beesley et al., 2000).

Scientific studies have shown that specific environmental factors can influence the onset of nereid reproduction:

According to global studies marine nereids are likely to induce epitoky in anticipation of a spawning event. Reproduction of these abundant but geographically separated individuals has evolved a method of synchronisation whereby spawning significantly increases fertilization success as well as survival and growth of new individuals (for more information see 'Behaviour'. 

Asexual reproduction:

Polychaetes are able to regenerate parts of their bodies which have been damaged or which are lost. They are readily able to regenerate appendages, palps, tails and most importantly their head. When the nerve cord is severed a new head will regenerate from the place where the cut was made. (for more information see 'Cell Biology').