An Example of Phylum Annelida: Nereis Dumerilii

In this article we will discuss about Nereis Dumerilii:- 1. External Structures of Nereis Dumerilii 2. Body Wall of Nereis Dumerilii 3. Body Cavity or Coelom 4. Digestive System 5. Respiratory System 6. Circulatory System 7. Excretory System 8. Nervous System 9. Reproductive System.

Contents:

1. External Structures of Nereis Dumerilii
2. Body Wall of Nereis Dumerilii
3. Body Cavity or Coelom of Nereis Dumerilii
4. Digestive System of Nereis Dumerilii
5. Respiratory System of Nereis Dumerilii
6. Circulatory System of Nereis Dumerilii
7. Excretory System of Nereis Dumerilii
8. Nervous System of Nereis Dumerilii
9. Reproductive System of Nereis Dumerilii

1. External Structures of Nereis Dumerilii:

The body of Nereis dumerilii is approximately 7-8 centime­tres in length. The colour is light violet and the regions of the body which are richly supplied with blood vessels appear reddish. The glittering appearance of the surface is due to the intersection of two sets of fine lines. The animal is long, narrow and cylin­drical (Fig. 17.2).

The body is divisible into about 80 segments or metameres and a distinct head is present at the anterior end. All the segments excepting the head and the last segment bear laterally placed, hollow, mus­cular and vertical, movable paired append­ages parapodia (Sing. Parapodium).

The ter­minal segment is termed as the anal seg­ment or pygidium and it bears at its poste­rior end a small round opening, the anus. Anal segment bears a pair of elongated anal cirri. On the ventral surface and near the base of the parapodium lies a nephridial aperture. Thus a pair of nephridiopores is present in each parapodial segment.

Structures in the Head:

The head is divisible into two parts: prostomium and peristomium (Fig. 17.3). Prostomium is an anterior, small, roughly conical lobe of the peristomium. It lies in front of the mouth. It is not a true body segment. The lobe bears tentacles, palps and eyes which serve as sensory organs. Peristomium is the first body segment which is ring-like and bears mouth ventrally.

The prostomium bears following structures:

(1) Prostomial Tentacles—paired, cylin­drical, small and placed in front.

(2) Palp—paired, elongated and compact and located after the tentalces.

(3) Eyes—two pairs, simple, round, pigmented and present on the dorsal side of the head.

The peristomium carries:

(1) Peristomial tentacles—four pairs, long, slender, cylindri­cal and laterally placed.

(2) Mouth—present on the ventral side as a transverse aperture.

Structures of the parapodium:

All the segments excepting the first and the last segments bear on either lateral side a fleshy, 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:

(1) Dorsally placed notopodium and

(2) Ventrally placed neuropodium (Fig. 17.4).

Both these parts are subdivided into lobes and both of them carry pack of needle-like structures, called setae (sing, seta) which project beyond the lobes.

Cirrus (PI. cirri):

The dorsal and ventral sides of the parapodium bear small, tentacle-­like, cylindrical appendages, called dorsal and ventral cirrus respectively. The ventral cirrus, situated ventral to the neuropodium, is smaller than the dorsal cirrus situated dorsally on the notopodium.

Setae:

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. One of the setae in each bundle is long, rod-like and dark in colour. It is known as aciculum and it projects on the surface.

Each seta consists of a basal shaft with which articulates a terminal blade.

Three types of setae are seen (Fig. 17.5):

(1) In one type the shaft is oar-shaped.

(2) In the second type shaft is slender and the blade is narrow, straight and elongated.

(3) In the other type the shaft is comparatively thicker and the blade is small and curved.

Functions:

The parapodia perform the following functions:

1. The parapodia are modified variously in different polychaetes and perform different locomotory functions, such as crawling and swimming.

2. The parapodia in some polychaetes are highly vascularized structure and function as respiratory organs.

2. Body Wall of Nereis Dumerilii:

The body wall is divisible into:

(a) Cuticle,

(b) Epidermis,

(c) Muscle layers and

(d) Pa­rietal epithelium (Fig. 17.6).

The details of each part are discussed below:

(a) Cuticle:

It is thin, slightly brittle and chitinous having a network of fine lines on the external surface which renders an ‘iridiscent lustre’. Numbers 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 cells. It is more thick on the ventral side specially near the parapodial joints. The dorsal side of the epidermis is richly sup­plied with blood vessels. Numbers of ‘twisted’ unicellular glands are present specially on the ventral side.

(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:

It 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.

3. Body Cavity or Coelom of Nereis Dumerilii:

The body of Nereis dumerilii is transverse section looks like a tube within a tube. The wall of the outer large tube is the body wall and the wall of the inner small tube is the gut wall. In between the two walls, lies a spacious cavity called coelom or body cavity, which is filled up with a fluid.

The coelom is thus lined externally by parietal epithelium (in­ner layer of the outer tube) and internally by visceral or splanchnic epithelium (outer layer of the inner tube). The coelom is di­vided into compartments by transverse partitions, called septa. The wall of each septum is perforated through which coelomic fluid communicates from one chamber to the other.

4. Digestive System of Nereis Dumerilii:

The alimentary canal begins from mouth and runs straight to end in another aperture, called anus. The presence of two openings (mouth and anus), for inlet and outlet respec­tively, shows a marked advancement over Platyhelminthes where only one aperture served the double purpose.

The gut wall exhibits following histological structures:

(1) Outer visceral epithelium;

(2) Next one layer of longitudinal muscles followed by another layer of circular muscles; and

(3) Innermost layer of enteric epithelium.

The alimentary canal consists of following parts:

(a) Mouth:

It is present on the ventral side of the peristomium as a transverse ap­erture and opens to 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 the pharynx.

(c) Pharynx:

It extends up to the fourth segment and is also lined internally by cuti­cle. One pair of cuticular teeth is enlarged to form jaws at the posterior end of pharynx. The jaws are extended along the longitudinal axis of the body and are round at the base and pointed at the apex.

The base is pro­vided with muscular attachment while the inner edge of the apex is serrated. 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 the oesophagus.

(d) Oesophagus:

It traverses through next five segments and receives a pair of large glandular caeca. The oesophagus com­municates with the intestine.

(e) Intestine:

It is a more or less straight tube which is constricted at each segment. The constrictions are intense at the posterior end.

(f) Rectum:

In the last segment, intestine continues as rectum. It is lined internally with cuticle and opens to the exterior through an aperture, called anus.

Developmentally, the buccal cavity, pha­rynx and rectum originate from the ectoderm which also forms the outer covering. And for this reason, like the outer wall, these are also lined by cuticle.

Food and Mechanism of Feeding:

Nereis dumerilii is carnivorous and devours small animals like crustacea and small molluscs.

It seizes the food by means of jaws and teeth. The entire bucco-pharyngeal region during capture of prey is everted out. The eversion is caused by the pressure of coelomic fluid and contraction of protractor muscles. When this happens the buccal cav­ity becomes turned inside out while the pharynx is thrown forward so that the jaws are opened and come to lie in front of the head.

The folding in of the buccal cavity and pharynx is caused by contraction of retractor muscles and relaxation of protractor mus­cles. Another type of Nereis (Nereis diversicolor) exhibits two different mecha­nisms of food-capture. Sometimes the ani­mal comes out of its burrow and ingests small and nutritious particles from the sur­face of mud.

On other occasions, it remains within the burrow and secretes mucus. Then by the undulations of the body it draws a current of water into the burrow. The mucous cone acts as a sort of net where small parti­cles carried in with water are strained off. Then, at intervals, the animal engulfs the net.

5. Respiratory System of Nereis Dumerilii:

Gills are absent in Nereis dumerilii. The function of respiration is taken over by the lobes of parapodia and dorsal integument. For this reason, these regions are richly supplied with blood vessels.

The physiology of respiration is known in Nereis virens. It lives at a depth of 20-30 cm and in almost oxygen-free sand. The animals draw water by producing water current while gaseous exchange takes place through the vascularised regions of the parapodia.

When the oxygen pressure in surrounding water is equal to the oxygen pressure of blood, gas­eous exchange ceases. This arrest of respira­tory exchanges is possible by restricting the blood flow only to the dorsal and ventral vessels. Nereis dumerilii draws nearly 75% of the oxy­gen from water.

6. Circulatory System of Nereis Dumerilii:

Blood of Nereis dumerilii is red in colour. The constituents of blood are plasma and corpus­cles. Haemoglobin remains dissolved in plasma and its quantity is 8-9 mg per cubic millimetre. The blood flows through definite blood vessels (Figs. 17.8 & 17.9).

The chief blood vessels are:

A. 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 distributory vessel running mid-ventrally from one end of the body to the other below the alimentary canal. It conveys blood from anterior to posterier end.

3. Neural blood vessel:

This is a delicate longitudinal vessel accompanying the ven­tral nerve cord.

B. Transverse or commissural vessels:

This dorsal vessel is connected to the ventral vessel in each 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.

C. Segmental intestinal vessels:

The ventral vessel gives off two pairs of intestinal vessels in each segment to form capillary network in the gut wall. From there blood is returned to the dorsal vessel by another two pairs of 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. Dorsal vessel is the main collecting vessel and blood flows through it from posterior to anterior end. Whereas the flow of blood is in opposite direction through ventral vessel and by trans­verse and intestinal vessels it sends blood to the different parts of the body.

7. Excretory System of Nereis Dumerilii:

The excretory system consists of series of metamerically arranged paired tubes, called nephridia or segmental organs. They are absent in the anterior and posterior seg­ments.

A nephridium is made up of three parts:

(a) The body

(b) Anterior prolonga­tion of the body and

(c) Coiled ciliated tube

(a) Body:

The body is irregular in outline, oval in shape and lies at the base of the parapodium in a transverse fashion.

(b) Anterior prolongation:

It is a con­tinuation of the inner end of the body. The narrow prolongation is almost equal in length to the body and runs forward and inward to become attached to mesentery.

(c) Coiled ciliated tube:

The coiled cili­ated tube is housed in the body cavity and is the anterior prolongation of the nephrid­ium (Fig. 17.10).

The external opening of the tube is called nephridiopore. It is a small, circular opening on the ventral surface of the body and at the base of the ventral cirrus of the parapodium.

The diameter of the aperture may be ex­tended or contracted. The nephridiopore leads into the tube which is ciliated for the most part. The tube extends up to the tip of anterior prolongation and then takes a sharp turn to run into the body of nephridium.

Inside the body, the tube follows a zigzag course and ultimately passes through the anterior prolongation to open as the nephhrostome into the preceding segment. The nephrostome is funnel-shaped and its border is beset with a number of narrow ciliated processes.

Dorsal Ciliated Organs:

Each segment bears on the dorsal surface specially devel­oped ciliated tract of coelomic epithelium in the form of short funnels without external aperture. These are called dorsal ciliated organs. The specific roles of these structures are not clearly understood. Some believe that they are excretory in function while others consider them as genital ducts of temporary nature.

8. Nervous System of Nereis Dumerilii:

The nervous system of Nereis dumerilii consists of:

(A) Central nervous system,

(B) Visceral nervous system and

(C) Sense organs.

A. Central nervous system:

It includes

(1) Cerebral ganglia or brain:

It is present in the prostomial region as a large bilobed mass (Fig. 17.11A). The brain contains spe­cialised cells, which produce hormone to speed up regeneration, It has also been ex­perimentally demonstrated that extirpation of brain leads to precocious sexual maturity.

(2) Oesophageal connectives:

Two stout nerves, each originating from the posterior region of the brain turn around the two sides of the mouth and unite on the ventral wall of the pharynx.

(3) 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. The individual ganglion is also formed by the fusion of two ganglia.

(4) Peripheral nerves:

These are 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.

B. Visceral nervous system:

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.

C. Sense organs.

Following sense organs are present in Nereis dumerilii:

(a) Eyes:

There are two pairs of eyes. Each eye is a cup-shaped and darkly pigmented structure. The concave side bears the retina, a circular aperture, pupil and a lens of gelatinous consistency (Fig. 17.11B).

Many elongated and slender cells which are arranged parallelly form the wall of the cup. These cells through the union of their outer ends form the optic nerve and their inner ends extend towards the lens as clear and hyaline rods. The region of the cuticle which covers the eyes, acts as the cornea.

(b) Olfactory organs:

The olfactory or­gans are known as nuchal organs. These paired organs are present on the posterior and dorsal side of the prostomium and re­main in close contact with the hinder part of the brain. Each nuchal organ has two pits lined with ciliated epithelium.

(c) Tactile organs:

The tentacles, palpi and cirri are regarded as specialised tactile sense organs. With the help of specialised sensory cells they can discriminate the changes in the environment.

9. Reproductive System of Nereis Dumerilii:

Sexes are separate in Nereis dumerilii but well- formed gonads in the form of testes or ova­ries are not regularly recognised. The go­nads develop by the proliferation of coelomic epithelial cells of the body cavity. The gonads are temporary structures and appear only in the breeding season.

The males de­velop only a pair of testes which are present in any one of the segments between nine­teenth and twenty-fifth. The number of tes­tes may be more in other species. During breeding season groups of cells pinch off from the testes into the coelomic fluid. These cells undergo division and each daughter cell develops into a sperm. The sperms have rod-shaped heads and vibratile tails.

Spherical ovaries in females appear along the entire length of the body. They are metamerically arranged and occur one pair in each segment. The ova (when young) be­come detached from the ovaries into the coelomic fluid where they attain maturity. Both ovaries and testes degenerate after the liberation of sex cells.

Mature reproductive cells are liberated probably through temporary apertures formed by the rupture of the body wall. Ferti­lization is external and occurs in sea water.

Structural changes during gonad forma­tion:

The formation of gametes induces changes in the posterior half of the body. Such changes are noted in the critical ap­pearance of lobes of the parapodia and in the number of setae in bundles. In addition, the prostomial eyes become enlarged and the terminal segment produces sensory papillae. The worms after such transformations are known as Heteronereis.

The heteronereis forms are free-swim­ming. The body is divisible into two distinct parts. The anterior or asexual part is called ‘Atoke’ and the posterior or sexual part is called ‘Epitoke’.

The changes of the parapodia in the posterior half of the body are, first, increase in size and secondly, the formation of leaf-like outgrowths on the lobes. Bristles which replace setae remain inserted into the parapodium and assume fan-like appearance (Fig. 17.4B).

The transformation to the heteronereis form is due to the impact of hormones, released into the blood plexus from certain specialised cells, which remain very close to the brain. Some authors, however, consider Nereis and Heteronereis as two distinct species.

Development:

The matured egg has two enveloping membranes. The outer cover is thin and the inner one is broad with radial striations. With these membranes the egg remains within a covering of gelatinous con­sistency. Oil droplets and yolk bodies remain scattered throughout the cytoplasm of the egg.

The fertilization or the entry of sperm cell brings following changes in the egg:

(a) Inner radiated layer dissolves,

(b) Egg completes the maturation phase by liberat­ing two polar bodies,

(c) Egg exhibits irregu­lar amoeboid movement and

(d) Consider­able rearrangement of cytoplasmic particles occurs.

This results into the shifting of oil droplets and yolk spherules towards the centre, thus leaving a side with granular cytoplasm and nucleus.

The zygote finally assumes spherical shape and starts to divide. This is called cleavage. First two divisions produce four cells. These cells are called macromeres. One macromere becomes larger than the other three. These macromeres divide unequally in three sets and thus give rise to twelve micromeres.

At the end of first unequal di­vision of the macromeres, four micromeres of equal sizes are produced. The second unequal division of the macromeres pro­duces four more micromeres, but this time three are of same sizes and one is large. The third unequal division of the macromeres again results three more micromeres of same sizes and one large micromere.

During these unequal divisions of the macromeres, the micromeres are not produced at their tops. On the contrary, after first division the micromeres are pushed towards right, and then during next division they shift to the left and again to the right. Such arrangement gives rise to a spiral pattern. The two large micromeres are known as somatoblasts or mesentomeres.

The micromeres give rise to ectoderm, somatoblasts or mesentomeres result into mesoderm and macromeres pro­duce endoderm layer. The micromeres spread over the macromeres and push them and somatoblasts inside. Further development involves transformation of these cells into the various structures of larva. The larva is known as trochophore larva (Fig. 17.12).