Nereis or Neanthes is a marine polychaete annelid that lives in burrows in sea bottom and comes out in night to prey upon small animals. N. virens (Sandworm), N. pelagica (Type-species) and N. succinea (Clam worm) are common species of Nereis.
Body of Nereis is dorsoventrally flattened, segmented and 30-40 cm long having 80 to 120 segments and divisible into acron, trunk and pygidium. Acron is divisible into anterior prostomium and posterior peristomium and a mouth in between on the ventral side. Trunk is metamerically segmented and segments are called metameres or somites each one of which bears a pair of parapodia. The last segment is called pygidium that bears an anus slightly on the dorsal side and a pair of thread like anal cirri and several minute sensory papillae on the dorsal side.
PARAPODIUM
Parapodia are paired locomotory organs of the body attached on the lateral side of each trunk segment. Each parapodium is made of two lobes, viz. dorsally placed notopodium and ventrally placed neuropodium. Both lobes carry thread-like appendages called dorsal and ventral cirri. The two lobes of parapodium are supported internally by two chitinous rods known as aciculum, on the apical part of which there are bundles of long, chitinous setae or chaetae which project beyond the outer margin. Nephridiopore is situated on the parapodium near the ventral cirrus.
SKIN
Skin is made of tall columnar cells and scattered glandular and sensory cells and is richly supplied with blood vessels. Outer surface of epidermis is coated with a layer of tough cuticle which carries a number of small pores for glands. Below the epidermis there are circular and longitudinal muscles in bundles and some oblique strands that help to carry out lateral undulation of body.
COELOM
The coelom is schizocoelom but true perivisceral cavity that is made of an outer parietal peritoneum and an inner visceral peritoneum that surrounds the alimentary canal. Coelom is divided into compartments by intersegmental septa and is filled with coelomic fluid containing amoeboid coelomocytes.
LOCOMOTION
Nereis can crawl, burrow and swim actively. Crawling is done by parapodia, while swimming involves lateral undulations of body, brought about by wave-like contractions of longitudinal muscles and use of parapodia in oar-like fashion.
DIGESTIVE SYSTEM
Alimentary canal is a straight tube extending from anterior to posterior end of the body. The anterior opening is the mouth and posterior opening the anus. Mouth is located on ventral side of peristomium and opens into the buccal cavity, which carries teeth or dentacles. Pharynx is a large chamber and is lined internally by cuticle. One pair of jaws is present at the posterior end of pharynx. Pharynx can be protruded out of mouth by protractor muscles and can be withdrawn by retractor muscles. Oesophagus occupies five segments and receives a pair of glands. It communicates with stomach–intestine, which is a more or less straight tube that is constricted in each segment. A distinct stomach is absent in Nereis. Epithelial lining of mid-gut contains gland cells which secrete digestive enzymes. Rectum is the last part of intestine and opens to outside by anus. Nereis is a carnivore and feeds on small animals such as crustaceans, molluscs, sponges and other animals.
Prey is captured by the eversion of pharynx, which brings the jaws in front to grasp the prey. Retraction is caused by contractions of retractor muscle which brings the prey deep into the pharynx. The ingested food is masticated in the buccopharyngeal region by denticles. Food passes through the intestine by peristalsis and digestion is mainly extracellular and the food is digested by the digestive juices secreted by the oesophageal glands and the glands in the epithelial lining of stomach-intestine. Absorption of digested food also occurs in the stomach-intestine. The undigested food passes into rectum from where it is egested through anus.