RESPIRATORY SYSTEM
The parapodia having blood capillary network and body wall carry out respiration. Oxygen diffuses from the surrounding water into the blood through the integument or parapodial surface due to great partial pressure in comparison to blood. Similarly from the blood carbon dioxide diffuses into the surrounding water.
BLOOD VASCULAR SYSTEM
Blood vascular system is closed type consisting of blood and blood vessels. There is no heart. The red colour of the blood is due to the respiratory pigment erythrocruorin dissolved in its plasma. Dorsal blood vessel serves as the main collecting vessel and runs mid-dorsally from one end of the body to the other end above the alimentary canal. It carries blood from posterior to anterior. Ventral blood vessel is the main distributary vessel running mid-ventrally from one end of the body to the other below the alimentary canal. It carries blood from anterior to posterior end. The dorsal vessel is connected to the ventral vessel in each segment by two pairs of transverse vessels through the capillary network. The ventral vessel gives off two pairs of ventral intestinal vessels in each segment to form capillary network in the gut wall. From there blood is returned to the dorsal vessel by two pairs of dorsal intestinal vessels. Blood is circulated by wave-like contraction of the lining of dorsal blood vessel.
EXCRETORY SYSTEM
One pair nephridia are found in each segment. Each nephridium consists of a syncytial mass of connective tissue and a coiled excretory or nephridial tubule which opens at both ends lying within it. The nephridial tubule opens by a ciliated funnel or nephrostome in the coelom of anterior segment. Posteriorly the tube opens by a nephridiopore located ventrally at the base of parapodium.
The nitrogenous wastes are collected from coelomic fluid and blood by diffusion and passed by the cilia of nephridial tubules to the outside via the nephridiopores. Useful substances are reabsorbed by its cells and returned to the blood capillaries.
NERVOUS SYSTEM
Nervous system includes a large bilobed cerebral ganglion or brain on the dorsal side of pharynx which supplies nerves to sense organs located on the prostomium and peristomium. Two connectives join the brain with the subpharyngeal ganglion by encircling the pharynx from the two sides and joining on the ventral side. Ventral nerve cord originates from the ventral side of the pharynx, from the suboesophageal ganglion and runs posteriorly along the midventral line. The ventral nerve cord is made of two separate cords which are enveloped by a common connective tissue sheath. Along its path, the ventral cord possesses a ganglion in each segment from which nerves are supplied to all organs.
SENSE ORGANS
The prostomial tentacles, prostomial palpi and peristomial cirri on the acron are the tactile organs which are sensitive to touch. Nuchal organs are a pair of ciliated pits on the prostomium, which are chemoreceptors and perceive taste and olfactory sense.
There are four simple eyes on the dorsal surface of prostomium. Each eye has a cup that contains light sensitive retinal cells and pigmented cells. The external cuticle in the front part forms a transparent cornea. In the cup there is a large transparent, gelatinous refractive lens that focuses light rays on to the retinal cells. The retinal cells are joined to nerve fibres of the optic nerve. The eyes are photoreceptors but cannot form proper image and only help in detecting changes in the light intensity.
REPRODUCTIVE SYSTEM
Nereis is dioecious as sexes are separate. Gonads develop only during the breeding season, in the summer months. Gametes are released as spermatogonia in male and as oogonia in female into the coelomic cavity where they undergo maturation to develop into spermatozoa and ova, respectively. There are no gonoducts and mature sperms and ova are discharged to the outside in water either through nephridial tubules or by the rupture of body wall.
HETERONEREIS
It is sexually mature form of Nereis in which transformation in anatomy and behaviour of the animal takes place during breeding season.
The anterior one-third region of body is the asexual region or atoke while the sexually mature posterior region is called epitoke and the worm with these two regions is known as heteronereis and the phenomenon of transformation of Nereis into sexual form is known as epitoky.
Heteronereis, instead of creeping about on sea bottom or living in burrows, swims actively in surface waters. Body is divisible into an anterior asexual atoke and a posterior sexual epitoke. Parapodia of posterior sexual region become larger and develop flattened leaf-like outgrowths for better respiration. Their setae are replaced by oar-shaped setae arranged in a fan-like manner for swimming. Dorsal cirri become sensitive. The eyes become greatly enlarged and more sensitive. Peristomial cirri become longer. Intestine becomes atrophied and functionless. Pygidium develops specialised sensory papillae.
Sexually mature individuals or Heteronereis swarm to the surface of seawater in the night in order to shed sperms or ova. Both males and females swim rapidly in a circle, females releasing a substance called fertilium which attracts the males and stimulates shedding of sperms, which in turn excites the females to shed the eggs. Fertilization is external in sea water.
TROCHOPHORE LARVA
Eggs hatch into a larval stage called trochophore or trochosphere. This larva is ciliated, unsegmented and almost pear-shaped, pelagic creature. There is a sensory apical plate bearing a tuft of cilia. A ganglion is present beneath the apical organ. A preoral ciliated band called prototroch encircles the body anterior to mouch. Mouth is ventral and the digestive tract is complete. A postoral ciliated band or metatroch lies behind the mouth and help in locomotion and food gathering. In some species of Nereis, a typical trochophore larva does not occur and instead the larva which hatches out of the egg is advanced trochophore which is sometimes termed as nectochaete.
The trochosphere is pelagic and swims about by its ciliated bands. Then it sinks to the bottom and starts metamorphosing into adult. The apical organ forms the prostomium with brain, tentacles and eyes. The part immediately behind forms the peristomium. Posterior part develops segments of the body. Gradually the larva metamorphoses into a young worm which settles at the bottom of the sea and starts burrowing life.