GENERAL FEATURES and LIFE HISTORY of NEREIS (Clam Worm) (Neanthes)

Mob.no.--7004774289 B.Sc-Part-1 Group-A

GENERAL FEATURES AND LIFE HISTORY OF NEREIS (CLAM WORM) ( NEANTHES)

DR.SUJIT KUMAR DEPARTMENT OF ZOOLOGY ALLMA IQBAL COLLEGE BIHARSHARIF,NALANDA INTRODUCTION Nereis is a typical polychaete genus, living in burrows in sand or mud, often with clams, a reason for which its species are commonly known as clamworms or sandworms. Neanthes is one of the largest and most common typical marine annelid with most of the characteristics of the phylum. Nereis includes several species, of which more common are N.virens, N.cultrifera, N.dumerilli, N diversicolor, N pelagica and N succinea. The following account largely pertains to Nereis virens. Systematic Position

Phylum: Annelida

Class: Polychaeta

Subclass: Errantea

Family: Nereidae

Genus: Neanthes or Nereis

Species: Virens Neanthes is found on the sea-shore in the shallow water in rock crevices, hidden under the stone or sea weeds. Some live in tubular U-shaped burrows lined mucus in sand or mud at tide level. It is carnivorous and nocturnal. At night it keeps its head protruded out of the burrow in search of prey, which is usually a small crustacean, mollusc or annelid. When breeding period approaches it leaves the burrow and comes at the water surface to lend a pelagic life, and then it is called heteronereis.

It is cosmopolitan in distribution found abundantly in Europe, North Pacific, America, Alaska, and other places. 1. Shape, size and colour- The body is long, narrow, slender, bilaterally symmetrical, tapering posteriorly and relatively broad anteriorly. It is slightly flattened dorsoventrally, dorsal surface being convex, while ventral surface flat or even somewhat concave. It may range from 30-40 cm in length and 2 to 6 mm in width. Different species are differently coloured. Colour may vary with age and sexual maturity. 2. Segmentation –

Body of Nereis is metamerically divided into a number of metameres or segments arranged in a linear series. All the metameres are nearly alike except the last one which is rounded. Number of metameres is fairly constant for a species; about 80 in N.cultrifera and N.dumerilli and about 200 in N.virens.

3. Division of body- Body of Neanthes is divisible into three well marked regions; head, trunk and pygidium.

(a) Head- lies at the anterior end and consists of two main parts- Prostomium and Periosteum. Prostomium is an anterior narrow, nearly triangular fleshy outgrowth, situated mid-dorsally in front of the mouth. Peristomium is a large ring-like structure carrying ventrally the transverse mouth. Sensory appendages and organs of head - Various sensory appendages and organs found on the head of Neanthes are:

Adaptations in Tubicolous forms:

Tubicolous polychaetes are those which live inside the tube in mud or sand, they are permanent tube dwellers. Eg. Chaetopterus Terebella, Serpula, Sabella, Glycera and Amphitrite. Tubicolous annelids are capable of secreting adhesive cement like substance by which, sand and mud particles etc. can be bound together for constructing tube. In Amphitrite, Sabella and Terebella, trunk segments have mucous glands which lubricate the tube. (1). Feeding mechanism: Since, the animal live permanently within tubes, they cannot move about to hunt for their prey, so food catching becomes a problem for tubicolous forms. The adaptations are such that the food may be brought to the animal within the tube. The animals develop such structures which may create water current towards the tube. The water current with it carries micro-organisms inside the burrow, the tentacles and then move towards the mouth down in the ciliated groove. All the tubicolous annelids have such cilia, whose beating maintains constant water current through the tube, so that the micro-organisms brought along the water current can be ingested by the annelid. In forms like Terebella, Serpula and Sabella, the head has many appendages, each of which is provided with a longitudinal ciliated groove whose cilia beat to produce the water current. The feeding mechanism of Chaetopterus is especially very interesting. The parapodia setae become peg like or hook like for anchoring the animal to the wall of the tube. Isabella, the gills are covered with sticky juice in which the minute food particles get attached. In Myxicola, the gill filaments are joined by a thin sheet and form a semicircle at the apex, in which food is captured.

(2). Respiration: Since the animals are confined to the tube, the appendages of the head become modified to form palp and gill filaments which aid in breathing. (3). Reduction in Locomotion: Because of the absence of locomotion, locomotory organs, such as parapodia are lost.

(4). Reduction of Sense Organs: Sense organs are also comparatively reduced.

(5) Prey Catching Device: Since the parapodia are reduced, they cannot bring about locomotion effectively during the time when the burrowing annelid crawl freely on the sea bed. So eversible pharynx is provided with papillae and it helps in caterpillar like movement of the annelids. The eversible pharynx is an adaptation, which has a double function i.e., catching the prey and helping in locomotion. Adaptations in Burrowing Form :

The burrowing polychaetes lead a peculiar life where sometimes they crawl on the seabed and sometimes burrow in the soil. So they possess some characters specialized for the burrowing mode of life and some characters, which help in crawling. For example, Arenicola, Glycera, Amphitrite, Aphrodite.

(1) Body shape: For burrowing, the animals have smooth external surface and also external appendages has to be minimized so that there may be minimum friction during burrowing, thus the burrowing annelids have slender vermiform body. In Aphrodite, the body is short and bread with arched dorsal side and a flat ventral side which is annulated to form a creeping sole.

(2). Reduction of Locomotion and Sensory Organs: Parapodia are lost or reduced and they have very few setae. In Arenicola, posterior end is without parapodia. The head tentacles and the palps are also reduced.

Thus, the members of group Polychaeta have radiated into many forms adapted to swimming, burrowing and crawling mode of life. b) Trunk: Posterior to the head, the rest of the body which is metamerically segmented (having 80-120 segments) is called trunk. The segments are known as metameres or somites which bear a pair of parapodia.

(c)Pygidium: The last segment of the body called variously as tail, anal segment or pyridium, is elongated, swollen and bears a terminal anus. It bears no parapodia but has a pair of elongated anal cirri and several minute sensory papillae.

4.Parapodia - The Parapodia are primarily the organs of locomotion used both in creeping and in swimming. They are highly muscular, well vascularised and glandular structure. Structure of the Parapodium: The segments excepting the first and last segment either lateral side fleshly, flat and hollow parapodium. Largest parapodia are encountered in the middle segments of the body, then the size of the parapodia decreases towards the two ends. Each parapodium is biramous in nature. It consists of a basal part and two distal parts. The two distal parts are- (1) dorsally placed notopodium and (2) ventrally placed neuropodium. Both these parts are subdivided into two leaf-like lobes or ligulae, a dorsal superior ligula and a ventral inferior lingula. The dorsal and ventral sides of the parapodium bear small, tentacle-like, cylindrical appendages, called dorsal and ventral cirrus respectively. Each part is supported internally by a chitinous rod, known as aciculum. Each part also bears a bundle of long, fine, stiff, chitinous bristles, the setae or chaetae, which project beyond its margin.

Seta: The setae are stiff, needle-like chitinous rods which remain in bundle within a sac in the skin. The sac is known as setigerous sac. The entire bundle may be moved in various directions with the help of muscles. Each seta consists of a basal shaft with which articulates a terminal blade . Three types of setae are seen – In one type the shaft In the second type In the third type the is oar shaped. shaft is slender and shaft is comparatively the blade in narrow, thicker and the blade straight and is small and curved. elongated.

5. Nephridiopores: In each segment on the ventral side of the body near the bases of parapodia are found laterally a pair of minute openings, the excretory opening or nephridiopores by which excretory materials are removed. Body wall

The body wall is divisible into (a) cuticle, (b) epidermis, (c) muscle layers and (d) parietal epithelium. a) Cuticle: It is thin, slightly brittle and chitinous having a network of fine lines on the external surface which renders an ‘iridescent lustre’. Number of minute openings are present on the cuticle through which the epidermal glands open to the exterior. b) Epidermis: This is formed by a single layer of tall columnar supporting cells and some scattered glandular and sensory cells. It is thicker on the ventral side specially near the parapodia joints. The dorsal side of the epidermis is richly supplied with blood vessels. c) Muscle layers: The outer muscular layer is circularly arranged and the inner layer runs longitudinally. The longitudinal muscles are present in four bundles - two are dorso-lateral and the remaining two are ventro-lateral. The circular muscles of dorsal and ventral sides are interconnected by two strands of oblique muscles. d) Parietal epithelium \ Peritoneum: t is a part of the coelomic epithelium which lines the outer wall of the coelom or body cavity. It is made up of a single layer of cells. Functions of body wall.

Body wall of Nereis performs a variety of functions.

(i) Cuticle protects the body against desiccation and mechanical injuries.

(ii) Epidermis, being highly vascular, serves for respiration

(iii) Epidermal glands secrete mucus for lining the burrow so as to prevent it from collapsing.

(iv) Epidermal sensory cells are responsible for perceiving external stimuli.

(v) Musculature helps in various types of movements.

(vi) Setae serve for locomotion

Coelom

The coelom is an extensive perivisceral cavity having an outer parietal peritoneum and an inner visceral peritoneum which encloses the alimentary canal. The coelom is schizocoelic in annelids having been formed by splitting of the mesoderm into two layers. Coelom is divided into a linear series of compartments by intersegmental septa. The septa also have apertures, hence, the coelomic compartments communicate. Each septum has a double layer of coelomic epithelium containing muscles and connective tissue. The coelom is filled with a coelomic fluid containing amoeboid corpuscles or coelomocytes. During the breeding season reproductive cells in various stages of development are also found in coelomic fluid. The coelom communicates with the outside by nephridia and coelomoducts.

Functions of coelomic fluid. The functions of coelomic fluid may be counted as under:  It provides turgidity to the body which aids in locomotion.

 It helps in the protection of the body by absorbing external shocks, if any, and acts as hydraulic skeleton.

 It helps in the distribution of various nutritive materials and respiratory gases to the whole body.

 It helps in removing excretory wastes from the body. Locomotion

Nereis can crawl on substratum as well as swim actively. Crawling is affected by the activity of parapodia alone, while swimming involves parapodia activity as well as snake-Like lateral undulations of body which are brought about by the wave-like contractions of longitudinal muscles

Gray (1939) has described the pattern of parapodial activity during locomotion in Nereis. According to his observations, each parapodium behaves like a miniature paddle, alternately beating backwards (effective stroke) and forwards (recovery stroke). Opposite parapodia of adjacent segments always perform the same stroke at a time, while opposite parapodia of the sane segment perform reverse strokes at the same time. Movements of parapodia are controlled by oblique and parapodia muscles and coelomic fluid which may be forced into, or withdrawn from them.

Digestive System

It is a straight tube extending from anterior to posterior end of the body suspended in the body cavity of the dorsal mesentery. It is open at both ends; the anterior opening is the mouth and posterior opening the anus. It is divided into three distinct regions: (i) Stomodaceum or foregut: Comprising buccal cavity and pharynx.

(ii) Mesenteron or midgut: Comprising oesophagus and stomach - intestine

(iii) Proctodaeum or hindgut: Comprising rectum a) Mouth - It is present on the ventral side of peristomium as a transverse aperture and opens at the buccal cavity. b) Buccal cavity - It is a broad chamber with cuticular lining. The cuticles have been thickened to form teeth or denticles or paragnaths. The buccal cavity leads into pharynx. (c) Pharynx - It extends up to the 4th segment and is also lined internally by cuticle. One pair of cuticular teeth is enlarged to form jaws at the posterior end of pharynx. Special bands of protractor and retractor muscles are present in the pharyngeal region. The protractor muscles evert the buccal cavity and pharynx as proboscis and the retractor muscles withdraw it. The pharynx leads to oesophagus. (d) Oesophagus - It traverses through next five segments and receives a pair of the large glandular caeca. It communicates with stomach -intestine. (e) Stomach - Intestine - A distinct stomach is absent. However, intestine is a more or less straight tube which is constricted at each segment. Epithelial lining of mid-gut contains scattered gland cells which secrete digestive enzymes.

(f) Rectum - In the last segment, intestine continues as rectum. It opens to the exterior through an aperture called anus .

Mechanism of digestion - Neanthes is carnivorous and feeds on small animals such as crustaceans, molluscs, sponges, larvae and other worms and animals.

Feeding mechanism: Generally, the prey is captured by the eversion of buccal cavity and protrusion of pharynx. Protrusion of pharynx brings about the jaws in front to grasp the prey. The everted buccopharyngeal region forms a kind of proboscis or introvert. Buccal cavity is everted due to pressure of coelomic fluid, while protrusion of pharynx is due to contraction of protractor muscles. Retraction is effected by the contractions of retractor muscle. This retraction brings the jaws close and cross one another to hold the prey and to carry into the pharynx.

Digestion, absorption and egestion: The ingested food first of all undergoes mastication in the buccopharyngeal region as it is provided with numerous denticles meant to masticate the food. The masticated food is pushed onwards inside the gut by rhythmic waves of contraction passing over the wall of alimentary canal from anterior to posterior end.

Digestion is mainly extracellular and the food is digested by the digestive juices secreted by the esophageal glands and the gland cells of the epithelial lining of stomach-intestine. Absorption of digested food also occurs in the stomach-intestine by the diffusion. The undigested part of the food passes on to the rectum from where it is egested through the terminal anus situated on the posterior end of the anal segment.

Respiration

Gills or any other special organs of respiration are lacking in Neanthes. The parapodia with their rich blood supply and body wall with its plexus of blood vessels sub serve the function of blood respiration. Gaseous exchange takes place at the surface of these organs. Oxygen diffuses from the surrounding water into the blood through the integument or parapodia surface due to great partial pressure in comparison to blood. Similarly, from the blood carbon dioxide diffuses ino the surrounding water due to great partial pressure than the surrounding water. Blood Vascular System

Neanthes has a well-developed and closed type of circulatory system consisting of a fluid tissue, called blood which circulates throughout the body through a system of closed tubes or blood vessels. The blood vessels are filled with blood of bright red colour. The red colour of the blood is due to the presence of a respiratory pigment erythrocruorin, which is like hemoglobin in its plasma.

(1) Blood - It consists of fluid medium called plasma containing numerous nucleated, colourless amoeboid cells or corpuscles and dissolved hemoglobin. (2) Blood Vessels - The chief blood vessels are –

Longitudinal vessels: There are three longitudinal vessels running along the entire length of the body. These are: 1. Dorsal Blood vessel: This 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 end.

2. Ventral Blood vessel: It is the main distributary vessel running mid ventrally from one end of the body to the other below the alimentary canal. It conveys blood from anterior to posterior end. 3.Neural blood vessel: This is a delicate longitudinal vessel accompanying the ventral nerve cord.

Transverse or Commissural vessels: The dorsal vessel is connected to the ventral vessel in cache segment by two pairs of transverse vessels. But this link is not direct. Transverse vessels originating from the ventral vessel first give off branches to the parapodia, alimentary canal and adjoining parts. Some of these branches ramify to form networks of fine vessels inside the parapodial lobes and in the integument of the dorsal surface. Then from these extensive capillary networks, stout vessels are formed which open into the dorsal vessel. Segmental intestinal vessel: The ventral vessel gives off two pairs of ventro-intestinal vessels in cach segment to form capillary network in the gut wall. From there blood is returned to the dorsal vessel by another two pairs of dorso-intestinal vessels.

Mechanism of Blood circulation: Blood remains in constant circulation through the vessels by means of contractions which are peristaltic in nature. Waves of contractions transmit along the walls of the vessels to drive the blood. A series of ring-like muscle fibres round the walls of the blood vessels at short intervals aid in contraction. The contractions of the dorsal vessel are the most powerful. Excretory System Excretion is brought about by nephridia, one pair of which is found in each segment except a few anterior and a few posterior ones. Each nephridium is distinguished into a somewhat oval, curved body and a narrow neck. Body of nephridium consists of a syncytial mass of connective tissue, containing a coiled excretory or nephridial tubule which opens at both ends. The nephridial tubule passes through the system into the anterior segment where it opens by a ciliated funnel or nephrostome, Posteriorly the tube opens by contractile nephridiopore located ventraliy ai the base of parapodium. virgin of nephrostome is produced into a number of long delicate processes provided with cilia. A nephridium possessing such a ciliated nephrostome is called metanephridium. Physiology of excretion- The chief of nitrogenous waste in polychaetes is ammonia (ammonotelic). Nephridia collect waste from coelomic fluid and blood by diffusion, the cilia of nephridial tubules causes the liquid waste to pass out of nephridopores. Any useful substance entering the excretory tubule is reabsorbed by its cells and returned to the blood capillaries. This is known as selective reabsorption. The nephridia also possess the power of osmoregulation. It plays an important role in maintaining salt and water balance in the body of Neanthes.

Nervous System

The nervous system of Nereis consists of (A)-Central nervous system (B) Peripheral nervous system (C) Visceral nervous system and (D) Sense organs.

(A) Central Nervous System: It includes:

Cerebral Ganglia or Brain: It is present in the prostomial region as a large bilobed mass. The brain contains specialized cells, which produce hormone to speed up regeneration.

Oesophageal Connectives: Two stout nerves each originating from the posterior region of the brain turns around the two sides of the mouth and units on the ventral wall of the pharynx.

Ventral Nerve cord: It originates from the ventral side of the pharynx, i.e. the region where two oesophageal connectives meet, and it runs posteriorly along the mid-ventral line. The ventral nerve cord is formed of two separate cords which are enveloped by a common connective tissue sheath. Along its path, the cord possesses a ganglion in each segment. (B) Peripheral Nervous System:

This system comprises of nerves given off by brain, oesophageal connectives and ganglia of the ventral nerve cord. From brain, nerves are supplied to the tentacles, palpi and eyes. The oesophageal connectives supply branches to innervate peristomeal tentacles. The ganglion on the ventral nerve cord sends nerves to the various parts of the corresponding segment.

In addition to the nerves belonging to the central nervous system another set of nerves is given off from the brain. These fine nerves with ganglia innervate the anterior part of the alimentary system. It is known as stomatogastric or visceral nervous system. (D) Sense Organs:

In neanthes the sense organs are specialized and well developed. On the basis of the function they are of the following types:

Tactile sense organs: The prostomial tentacles, prostomial palpi and peristomial cirri of head are the main tactile sense organs as they are sensitive to touch.

Chemoreceptors: Nuchal organs are a pair of pits on the prostomium, lined with ciliated columnar epithelium with some gland cells. These organs are of doubtful nature but some have regarded them as chemoreceptor and olfactory sense organs as being sensitive to taste and smell respectively. Photoreceptors: There are four simple eyes on the dorsal surface of prostomium which are sensitive to light. Each eye has a cup made of pigmented retinal cells produced inwards into rods, the opening of the cup is a pupil. In fact, each retinal cell has three distinct parts: an outer nucleated part with nerve fibres, a highly pigmented main body and an inner part of transparent cuticular rod. The external cuticle forms a transparent cornea. Inside the cup is a transparent, gelatinous, refractive lens. The retinal cells are joined to nerve fibres of the optic nerve. The eyes are photoreceptive. They are not related to the formation of image but help in detecting changes in light intensity. Reproductive System

1. Gonads - Most species of Nereis are dioecious (unisexual). Sexes are separate. Gonads (testes and ovaries) are neither distinct nor permanent organs. They are seasonal and develop only during the breeding season i.e, in the summer months. Gametes are shed as spermatogonia in male and as oogonia in female into coelomic fluid where they undergo maturation to develop into spermatozoa and ova, respectively. Spermatozoa or sperms are small cells with a minute rod-shaped head and a long vibratile tail. Ova are somewhat large and rounded and packed with yolky globules. 2. Gonoducts - In Nereis there are no gonoducts Ripe sperms and ova are discharged to outside sea water mostly through metanephridia by the action of cilia borne by nephrostomes and nephridial tubules.

3. Dorsal ciliated organs - A pair of dorsal ciliated organs occurs in each segment in close relation to the dorsal longitudinal muscles. It has been said that during sexual maturity, funnels open to the exterior by temporarily acquired minute apertures opposite to them in body wall. However, the real function of ciliated funnels remains controversial. Heteronereis Epitoky-

At sexual maturity, most of the posterior segments, filled with gametes, exhibit morphological and anatomical differentiation. These constitute the sexual region or epitoke of worm. Few anterior segments, which do not take part in gamete-formation, constitute the asexual region or atoke. Sexually mature worm with these two regions is known as heteroncreies and the phenomenon involving transformation of non-sexual individual into sexual individual is referred to at epitoky. Characteristic features of Heteronereis :  Instead of creeping about on sea bottom or living in burrows like Nereis, the heteronereis swims actively in surface waters.

 Body of heteronereis is divisible into two distinct regions: an anterior sexual atoke, and a posterior sexual epitoke.

 Parapodia of posterior sexual region becomes larger, more vascularized and develop flattened leaf-like outgrowths for more rapid respiration. Their normal setae are replaced by flattened oar-shaped setae arranged in a fan-like manner, to offer a larger surface for swimming. Dorsal cirri are altered. Leucocytes break down and digest the original parapodia muscles and new muscles are formed.

 The eyes become greatly enlarged and conspicuous.

 Prostomial palps and tentacles become reduced, but peristomial cirri become longer

 Intestine becomes compressed and functionless due to much developed gonads.

 Sensory projections become shrunken while pygidium or anal segment develops special sensory papillae.

 In some species, such as Nereis virens, heteronereis shows sexual dimorphism, male having less unaltered anterior segments than female. Significance of Heteronereis:

Nereis is the non-sexual phase and lives in the burrow of creeps at the bottom of the sea, while Heteronereis is the sexual phase that wins actively at the surface of sea-water. So, it discharges gametes to far off places in the sea and, thus, helps in the dispersal of species.

Life History of Nereis Swarming Sexually mature individuals or Heteronereis swim to the surface of seawater in order to shed sperms or ova. This behavior is called swarming. It generally occurs at night and some species, while swarming, perform a nuptial dance in which both males and females swim rapidly in a circle. Female produce a substance, called fertilium, which attracts the male and stimulates shedding of sperms, which in turn excites the female and stimulates shedding of eggs. Fertilization-Fertilization is external and takes place in open sea water.

Development- Development of Nereis comprises three distinct periods-

(a) Prelarval period – • The newly discharged egg of neanthes is spherical in shape and possesses two membranes; a thin outer membrane which is delicate, followed by a thick radially striated membrane called zona radiata which encloses the ooplasm having yolk spherules, oil droplets and female nucleus. An egg is surrounded in a thick, transparent, gelatinous coat. However, immediately after fertilization, several changes occur in an egg- the zona radiata disappears, yolk spherules aggregate at the lower end of the egg leaving a clear zone in the upper end. The lower end, thus, becomes to be known as vegetal pole and the upper end as animal pole. The egg is, thus, telolecithal. Two small polar bodies are extruded towards the animal pole and the zygote undergoes cleavage. • First two cleavages of zygote are equal and vertical and result in four cells or blastomeres, lying in the same plane. Cleavage is determinate, i.e. fate of blastomere is fixed and after four-cell stage, each blastomere gives rise to only one quadrant of embryo.

• Third cleavage is unequal and horizontal, i.c. at right angles to the first two cleavages. It produces four small yolk-free micromeres towards the animal pole and four large, yolky macromeres towards the vegetal pole.

• Fourth fifth and sixth cleavages are also horizontal and cut off three more quartettes lie alternately. This type of cleavage is termed spiral cleavage. One micromere of second quartette and one of fourth are large than the others. These are respectively termed the first and second somatoblasts. Micromeres (except the second somatoblast) are the forerunners of ectoderm, meagameres of endoderm, and second somatoblast of mesoderm. Cleavage results into a stereoblastula. Gastrulation takes place by invagination of the gut pouch resulting in a ciliated gastrula.

(b)Larval period (trochophore): After gastrulation, ciliated embryo rapidly develops into a larval stage, called trochophore or trochosphere. Structure of trochophore- The Trochophore larval stage is not only characteristic of Polychaete, but it also occurs in Mollusca, Archiannelida, and polyclad Turbellaria. It is a minute, ciliated, unsegmented, and almost pear-shaped pelagic creature, with oral and aboral surfaces recognizable. In a full grown typical trochophore larva, there is a sensory apical organ or plate bearing a tuft of cilia. Brain rudiments, as a ganglion, are usually evident beneath the apical organ. A characteristic feature of trochophore is the presence of a preoral ciliated girdle of cells, just above equator, and called prototroch. Digestive tract is complete. Mouth lies ventrally just beneath the prototroch and anus is near the lower apex A postoral ciliated band or metatroch lies behind the mouth. These ciliated rings help in feeding and locomotion. Gut is regionated into oesophagus, stomach and intestine. Mesoderm is a pair of undifferentiated masses of cells located in the lower cone. Lying beside these is a pair of protonephridia that develop from ectoderm. Trochophore, at its early stage of development, lacks a coelom. Its body is composed primarily of an outer ectoderm with ectodermal derivatives (nervous tissue and scattered ectoderm elements), and inner endoderm forming the gut. Space enclosed between gut endoderm and ectoderm is called blastocoel. However, a typical trochophore larva does not occur in Nereis. Instead, trochophore stage is embryonic and passed inside the egg membrane. The larva which hatches out of the egg is advanced trochophore larva and is termed nectochaete.

The troposphere is pelagic. It drifts about in the sea swimming by its prototroch and organs of the adult begin to form. The apical organ forms the prostomium with brain, tentacles and eyes.

The part immediately behind forms the peristomium. The larva grows from the anal end as an elongated cylinder which forms segments of the body by metameric segmentation. The larval nephridia are replaced by permanent ones, larval setae are dropped, tentacles, palps and parapodia are formed. The advanced larva consists of the adult head and body segments which are drawn together and joined to metamorphose the larva into a young worm. The young worm, resulted, settles at the bottom of the sea and starts forming its burrow and gradually attains adulthood. THANK YOU