G E N E R A L F E A T U R E S A N D L I F E H I S T O R Y O F N E R E I S ( C L A M W O R M )

( N E A N T H E S )D R . S U J I T K U M A R

D E P A R T M E N T O F Z O O L O G Y

ALLMA IQBAL COLLEGEB I H A R S H A R I F , N A L A N D A

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

INTRODUCTIONNereis is a typical polychaete

genus, living in burrows in sand or mud, often withclams, a reason for which its species are commonlyknown as clamworms or sandworms. Neanthes is oneof the largest and most common typical marineannelid with most of the characteristics of the phylum.Nereis includes several species, of which morecommon are N.virens, N.cultrifera, N.dumerilli, Ndiversicolor, N pelagica and N succinea. The followingaccount 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 shallowwater in rock crevices, hidden under the stone or sea weeds. Somelive in tubular U-shaped burrows lined mucus in sand or mud attide level. It is carnivorous and nocturnal. At night it keeps its headprotruded out of the burrow in search of prey, which is usually asmall crustacean, mollusc or annelid. When breeding periodapproaches it leaves the burrow and comes at the water surface tolend a pelagic life, and then it is called heteronereis.

It is cosmopolitan in distribution found abundantly inEurope, 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 mayrange from 30-40 cm in length and 2 to 6 mm in width. Differentspecies are differently coloured. Colour may vary with age andsexual maturity.

2. Segmentation –

Body of Nereis is metamerically divided into a number of metameres orsegments arranged in a linear series. All the metameres are nearly alike exceptthe last one which is rounded. Number of metameres is fairly constant for aspecies; 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 orsand, they are permanent tube dwellers. Eg. Chaetopterus Terebella, Serpula,Sabella, Glycera and Amphitrite.Tubicolous annelids are capable of secreting adhesive cement like substance bywhich, sand and mud particles etc. can be bound together for constructing tube. InAmphitrite, Sabella and Terebella, trunk segments have mucous glands whichlubricate 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 fortubicolous forms. The adaptations are such that the food may be brought to theanimal within the tube. The animals develop such structures which may createwater current towards the tube. The water current with it carries micro-organismsinside the burrow, the tentacles and then move towards the mouth down in theciliated groove. All the tubicolous annelids have such cilia, whose beating maintainsconstant water current through the tube, so that the micro-organisms broughtalong the water current can be ingested by the annelid.

In forms like Terebella, Serpula and Sabella, the head has manyappendages, each of which is provided with a longitudinal ciliatedgroove whose cilia beat to produce the water current. The feedingmechanism of Chaetopterus is especially very interesting. Theparapodia setae become peg like or hook like for anchoring the animalto the wall of the tube. Isabella, the gills are covered with sticky juicein which the minute food particles get attached. In Myxicola, the gillfilaments 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, theappendages of the head become modified to form palp and gillfilaments 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 comparativelyreduced.

(5) Prey Catching Device: Since the parapodia are reduced, they cannotbring about locomotion effectively during the time when the burrowingannelid crawl freely on the sea bed.So eversible pharynx is provided with papillae and it helps in caterpillarlike movement of the annelids. The eversible pharynx is an adaptation,which has a double function i.e., catching the prey and helping inlocomotion.

Adaptations in Burrowing Form :

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

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

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

Thus, the members of group Polychaeta have radiated into many forms adapted toswimming, burrowing and crawling mode of life.

b) Trunk: Posterior to the head, the rest of the body which ismetamerically segmented (having 80-120 segments) is calledtrunk. The segments are known as metameres or somites whichbear 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 aterminal anus. It bears no parapodia but has a pair of elongatedanal cirri and several minute sensory papillae.

4.Parapodia - The Parapodia are primarily the organs oflocomotion used both in creeping and in swimming. They arehighly muscular, well vascularised and glandular structure.

Structure of the Parapodium: The segments excepting the first and last segmenteither lateral side fleshly, flat and hollow parapodium. Largest parapodia areencountered in the middle segments of the body, then the size of the parapodiadecreases towards the two ends. Each parapodium is biramous in nature. Itconsists of a basal part and two distal parts. The two distal parts are- (1) dorsallyplaced 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 dorsaland ventral sides of the parapodium bear small, tentacle-like,cylindrical appendages, called dorsal and ventral cirrusrespectively. 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 projectbeyond its margin.

Seta: The setae are stiff, needle-like chitinous rods which remainin bundle within a sac in the skin. The sac is known as setigeroussac. The entire bundle may be moved in various directions withthe help of muscles. Each seta consists of a basal shaft with whicharticulates a terminal blade . Three types of setae are seen –

In one type the shaft is oar shaped.

In the second type shaft is slender and the blade in narrow, straight and elongated.

In the third type the shaft is comparatively thicker and the blade is small and curved.

5. Nephridiopores: In each segment on the ventral side ofthe body near the bases of parapodia are found laterally apair of minute openings, the excretory opening ornephridiopores by which excretory materials are removed.

Body wall

The body wall is divisible into (a) cuticle, (b) epidermis, (c) musclelayers and (d) parietal epithelium.

a) Cuticle: It is thin, slightly brittle and chitinous having a network of fine lineson the external surface which renders an ‘iridescent lustre’. Number ofminute openings are present on the cuticle through which the epidermalglands open to the exterior.

b) Epidermis: This is formed by a single layer of tall columnar supporting cellsand some scattered glandular and sensory cells. It is thicker on the ventralside specially near the parapodia joints. The dorsal side of the epidermis isrichly supplied with blood vessels.

c) Muscle layers: The outer muscular layer is circularly arranged and the innerlayer runs longitudinally. The longitudinal muscles are present in fourbundles - two are dorso-lateral and the remaining two are ventro-lateral.The circular muscles of dorsal and ventral sides are interconnected by twostrands of oblique muscles.

d) Parietal epithelium \ Peritoneum: t is a part of the coelomic epitheliumwhich lines the outer wall of the coelom or body cavity. It is made up of asingle 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 itfrom 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 parietalperitoneum and an inner visceral peritoneum which encloses the alimentarycanal. The coelom is schizocoelic in annelids having been formed by splitting ofthe mesoderm into two layers. Coelom is divided into a linear series ofcompartments by intersegmental septa. The septa also have apertures, hence, thecoelomic compartments communicate. Each septum has a double layer ofcoelomic epithelium containing muscles and connective tissue. The coelom isfilled with a coelomic fluid containing amoeboid corpuscles or coelomocytes.During the breeding season reproductive cells in various stages of developmentare also found in coelomic fluid. The coelom communicates with the outside bynephridia and coelomoducts.

Functions of coelomic fluid. The functions of coelomic fluid may be counted asunder:

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 actsas hydraulic skeleton.

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

It helps in removing excretory wastes from the body.Locomotion

Nereis can crawl on substratum as well as swim actively. Crawling is affectedby the activity of parapodia alone, while swimming involves parapodia activity as well assnake-Like lateral undulations of body which are brought about by the wave-likecontractions 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 miniaturepaddle, alternately beating backwards (effective stroke) and forwards(recovery stroke). Opposite parapodia of adjacent segments alwaysperform the same stroke at a time, while opposite parapodia of the sanesegment perform reverse strokes at the same time. Movements ofparapodia are controlled by oblique and parapodia muscles and coelomicfluid which may be forced into, or withdrawn from them.

Digestive System

It is a straight tube extending from anterior to posteriorend of the body suspended in the body cavity of the dorsal mesentery. It isopen at both ends; the anterior opening is the mouth and posterioropening 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 atransverse aperture and opens at the buccal cavity.

b) Buccal cavity - It is a broad chamber with cuticular lining. Thecuticles have been thickened to form teeth or denticles orparagnaths. 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 pharyngealregion. The protractor muscles evert the buccal cavity and pharynx as proboscis andthe retractor muscles withdraw it. The pharynx leads to oesophagus.

(d) Oesophagus - It traverses through next fivesegments and receives a pair of the large glandularcaeca. It communicates with stomach -intestine.

(e) Stomach - Intestine - A distinct stomach is absent. However, intestine is amore or less straight tube which is constricted at each segment. Epitheliallining of mid-gut contains scattered gland cells which secrete digestiveenzymes.

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

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

Feeding mechanism: Generally, the prey is captured by the eversion of buccalcavity and protrusion of pharynx. Protrusion of pharynx brings about the jaws infront to grasp the prey.

The everted buccopharyngeal region forms a kind of proboscis or introvert. Buccalcavity is everted due to pressure of coelomic fluid, while protrusion of pharynx isdue to contraction of protractor muscles. Retraction is effected by the contractionsof retractor muscle. This retraction brings the jaws close and cross one another tohold the prey and to carry into the pharynx.

Digestion, absorption and egestion: The ingested food first of all undergoesmastication in the buccopharyngeal region as it is provided with numerous denticlesmeant to masticate the food. The masticated food is pushed onwards inside the gutby rhythmic waves of contraction passing over the wall of alimentary canal fromanterior to posterior end.

Digestion is mainly extracellular and the food is digested by the digestive juicessecreted by the esophageal glands and the gland cells of the epithelial lining ofstomach-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 egestedthrough the terminal anus situated on the posterior end of the anal segment.

Respiration

Gills or any other special organs of respiration are lacking inNeanthes. The parapodia with their rich blood supply and body wall with itsplexus of blood vessels sub serve the function of blood respiration. Gaseousexchange takes place at the surface of these organs. Oxygen diffuses from thesurrounding water into the blood through the integument or parapodia surfacedue to great partial pressure in comparison to blood. Similarly, from the bloodcarbon dioxide diffuses ino the surrounding water due to great partial pressurethan the surrounding water.

Blood Vascular System

Neanthes has a well-developed and closed type of circulatory systemconsisting of a fluid tissue, called blood which circulates throughout the bodythrough a system of closed tubes or blood vessels. The blood vessels are filled withblood of bright red colour. The red colour of the blood is due to the presence of arespiratory 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 servesas the main collecting vessel and runsmid-dorsally from one end of the bodyto the other end above the alimentarycanal. It carries blood from posterior toanterior end.

2. Ventral Blood vessel: It is the maindistributary vessel running mid ventrallyfrom one end of the body to the otherbelow the alimentary canal. It conveysblood from anterior to posterior end.

3.Neural blood vessel: This is a delicate longitudinalvessel accompanying the ventral nerve cord.

Transverse or Commissural vessels: The dorsal vessel isconnected to the ventral vessel in cache segment by twopairs of transverse vessels. But this link is not direct.Transverse vessels originating from the ventral vesselfirst give off branches to the parapodia, alimentary canaland adjoining parts. Some of these branches ramify toform networks of fine vessels inside the parapodiallobes and in the integument of the dorsal surface. Thenfrom these extensive capillary networks, stout vesselsare formed which open into the dorsal vessel.

Segmental intestinal vessel: The ventral vessel gives offtwo pairs of ventro-intestinal vessels in cach segment toform capillary network in the gut wall. From there blood isreturned to the dorsal vessel by another two pairs ofdorso-intestinal vessels.

Mechanism of Blood circulation: Blood remains inconstant circulation through the vessels by means ofcontractions which are peristaltic in nature. Waves ofcontractions transmit along the walls of the vessels to drivethe blood. A series of ring-like muscle fibres round thewalls of the blood vessels at short intervals aid incontraction. The contractions of the dorsal vessel are themost powerful.

Excretory SystemExcretion is brought about by nephridia, one pair of

which is found in each segment except a few anteriorand a few posterior ones. Each nephridium isdistinguished into a somewhat oval, curved body and anarrow neck. Body of nephridium consists of a syncytialmass of connective tissue, containing a coiled excretoryor nephridial tubule which opens at both ends. Thenephridial tubule passes through the system into theanterior segment where it opens by a ciliated funnel ornephrostome, Posteriorly the tube opens by contractilenephridiopore located ventraliy ai the base ofparapodium. virgin of nephrostome is produced into anumber of long delicate processes provided with cilia. Anephridium possessing such a ciliated nephrostome iscalled metanephridium.

Physiology of excretion- The chief of nitrogenous waste inpolychaetes is ammonia (ammonotelic). Nephridia collect wastefrom coelomic fluid and blood by diffusion, the cilia of nephridialtubules causes the liquid waste to pass out of nephridopores. Anyuseful substance entering the excretory tubule is reabsorbed by itscells and returned to the blood capillaries. This is known asselective reabsorption. The nephridia also possess the power ofosmoregulation. It plays an important role in maintaining salt andwater balance in the body of Neanthes.

Nervous System

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

(A) Central Nervous System: It includes:

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

Oesophageal Connectives: Two stout nerves each originating from theposterior region of the brain turns around the two sides of the mouth andunits 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 runsposteriorly along the mid-ventral line. The ventral nerve cord is formed oftwo separate cords which are enveloped by a common connective tissuesheath. Along its path, the cord possesses a ganglion in each segment.

(B) Peripheral Nervous System:

This system comprises of nerves given off by brain, oesophagealconnectives and ganglia of the ventral nerve cord. From brain, nerves aresupplied to the tentacles, palpi and eyes. The oesophageal connectivessupply branches to innervate peristomeal tentacles. The ganglion on theventral nerve cord sends nerves to the various parts of thecorresponding segment.

(C) Visceral nervous system:

In addition to the nerves belonging to the central nervous systemanother set of nerves is given off from the brain. These fine nerves withganglia innervate the anterior part of the alimentary system. It is knownas stomatogastric or visceral nervous system.

(D) Sense Organs:

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

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

Chemoreceptors: Nuchal organs are a pair of pits on the prostomium,lined with ciliated columnar epithelium with some gland cells. Theseorgans are of doubtful nature but some have regarded them aschemoreceptor and olfactory sense organs as being sensitive to tasteand smell respectively.

Photoreceptors: There are four simple eyes onthe dorsal surface of prostomium which aresensitive to light. Each eye has a cup made ofpigmented retinal cells produced inwards intorods, the opening of the cup is a pupil. In fact,each retinal cell has three distinct parts: anouter nucleated part with nerve fibres, a highlypigmented main body and an inner part oftransparent cuticular rod. The external cuticleforms a transparent cornea. Inside the cup is atransparent, gelatinous, refractive lens. Theretinal cells are joined to nerve fibres of theoptic nerve. The eyes are photoreceptive. Theyare not related to the formation of image buthelp 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 norpermanent organs. They are seasonal anddevelop only during the breeding season i.e,in the summer months. Gametes are shed asspermatogonia in male and as oogonia infemale into coelomic fluid where theyundergo maturation to develop intospermatozoa and ova, respectively.Spermatozoa or sperms are small cells with aminute rod-shaped head and a long vibratiletail. Ova are somewhat large and roundedand packed with yolky globules.

2. Gonoducts - In Nereis there are no gonoducts Ripesperms and ova are discharged to outside sea watermostly through metanephridia by the action of cilia borneby nephrostomes and nephridial tubules.

3. Dorsal ciliated organs - A pair of dorsal ciliated organsoccurs in each segment in close relation to the dorsallongitudinal muscles. It has been said that during sexualmaturity, funnels open to the exterior by temporarilyacquired minute apertures opposite to them in body wall.However, the real function of ciliated funnels remainscontroversial.

Heteronereis

Epitoky-

At sexual maturity, most of the posterior segments,filled with gametes, exhibit morphological and anatomicaldifferentiation. These constitute the sexual region or epitokeof worm. Few anterior segments, which do not take part ingamete-formation, constitute the asexual region or atoke.Sexually mature worm with these two regions is known asheteroncreies and the phenomenon involving transformationof non-sexual individual into sexual individual is referred toat 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 sexualepitoke.

Parapodia of posterior sexual region becomes larger, more vascularized and develop flattened leaf-likeoutgrowths for more rapid respiration. Their normal setae are replaced by flattened oar-shaped setaearranged in a fan-like manner, to offer a larger surface for swimming. Dorsal cirri are altered. Leucocytesbreak 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 unalteredanterior segments than female.

Significance of Heteronereis:

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

Life History of NereisSwarming Sexually mature individuals or Heteronereis

swim to the surface of seawater in order to shed sperms or ova. Thisbehavior is called swarming. It generally occurs at night and some species,while swarming, perform a nuptial dance in which both males and femalesswim rapidly in a circle. Female produce a substance, called fertilium, whichattracts the male and stimulates shedding of sperms, which in turn excitesthe female and stimulates shedding of eggs.

Fertilization-Fertilization is external and takes place in open sea water.

Development- Development of Nereis comprises three distinctperiods-

(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 thickradially striated membrane called zona radiata which encloses the ooplasmhaving yolk spherules, oil droplets and female nucleus. An egg is surrounded ina thick, transparent, gelatinous coat. However, immediately after fertilization,several changes occur in an egg- the zona radiata disappears, yolk spherulesaggregate at the lower end of the egg leaving a clear zone in the upper end. Thelower end, thus, becomes to be known as vegetal pole and the upper end asanimal pole. The egg is, thus, telolecithal. Two small polar bodies are extrudedtowards the animal pole and the zygote undergoes cleavage.

• First two cleavages of zygote are equal and vertical and result in four cells orblastomeres, lying in the same plane. Cleavage is determinate, i.e. fate ofblastomere is fixed and after four-cell stage, each blastomere gives rise to onlyone quadrant of embryo.

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

• Fourth fifth and sixth cleavages are also horizontal and cut off three morequartettes lie alternately. This type of cleavage is termed spiral cleavage. Onemicromere of second quartette and one of fourth are large than the others. Theseare 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. Cleavageresults into a stereoblastula. Gastrulation takes place by invagination of the gutpouch 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 ringshelp in feeding and locomotion. Gut is regionated into oesophagus, stomach andintestine. Mesoderm is a pair of undifferentiated masses of cells located in thelower cone. Lying beside these is a pair of protonephridia that develop fromectoderm. Trochophore, at its early stage of development, lacks a coelom. Its bodyis composed primarily of an outer ectoderm with ectodermal derivatives (nervoustissue 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, trochophorestage is embryonic and passed inside the egg membrane. The larva which hatches outof the egg is advanced trochophore larva and is termed nectochaete.

(c) Post-larval period or metamorphosis of Trochophore larva:

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

The part immediately behind forms the peristomium. The larva grows from theanal end as an elongated cylinder which forms segments of the body by metamericsegmentation. The larval nephridia are replaced by permanent ones, larval setae aredropped, tentacles, palps and parapodia are formed. The advanced larva consists ofthe adult head and body segments which are drawn together and joined tometamorphose the larva into a young worm. The young worm, resulted, settles at thebottom of the sea and starts forming its burrow and gradually attains adulthood.

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