ANNELIDA

TOPICS TO BE DISCUSSED

Introduction Unique characters General characters Morphology and anatomy Classification Harmful effects Benifits

INTRODUCTION

LINNAEUS placed all invertebrates except

insects in the taxon vermes.

LAMARCK established the taxon

annelida.

He coined the term annelida from the

latin word annulus( tiny ring).

Cuvier united the annelids and the

arthropods under the taxon articulata.

UNIQUE CHARACTERS

Cephalization.Metameric segmentation.Schizocelom(true coelom).Blood vascular system.

GENERAL CHARACTERS

They occur in all habitats(marine , freshwater, terrestrial).They are triploblastic( having three primary germ layers i.e. Outer ectoderm, middle mesoderm and inner endoderm).They are bilaterally symmetrical(body can be divided into two mirror images by the median sagittal plane).

They are schizocoelomate( true coelom) protostomes ( blastopore first develops into mouth).They exhibit homonomous metamerism(body is divided into linear series of similar segments , which are separated from one another externally by intersegment groove and internally by intersegment septa).Body is divided into three regions- prostomium – false segment. trunk – contains segments. pygidium –anal segment.Growth results from the addition of new segments from teloblastic growth zone located just in front of the pygidium.

MORPHOLOGY AND

ANATOMY INTEGUMENTARY SYSTEM:

Body wall consists of cuticle , epidermis , dermis ,

musculature( circular and longitudinal layers ) and a parietal

peritoneum.

Cuticle is produced by the underlying epidermis.

Epidermis is formed of one cell thick ectodermal

Epithelial cells, they are : Large gland cells- contains mucus and albumin secreting cells.Supporting cells-oval nucleus at the center and large in number.Basal cells-totipotent cells.Receptor cells- these are sensory in function.Chitinous bristles that project out from the epidermis are called setae(s- shaped) . They provide grip on the substratum.

COELOMCoelom(perivisceral cavity) is a apace between alimentary canal and body wall.Coelom acts as a hydrostatic skeleton as it helps in locomotion.Coelom consists of 4 types of cells: phagocytes-they have several membranous folds on the surface and phagocytic in nature.Chloragogen cells-contains yellowish granules called chloragosomes.Circular nucleated cells-has characteristic marking on their surface.Mucocytes-secrete mucus.

LOCOMOTION Locomotion has three stages-extension ,

contraction , and anchoring . Earthworms( on smooth surfaces) uses its

mouth as a sucker for locomotion.

EXCRETORY SYSTEM

o The excretory organs of earthworm are minute, coiled tube like structure called Nephridia.

o The nephridia are ectodermal in origin.

TYPICAL NEPHRIDIAo These are found in earthworms like Dravidia grandis.

o It consists of three parts. They are :

Nephrostome – ciliated funnel

Neck

Body of Nephridia

oThe body of nephridium is divide into :

Ciliated - intracellular in origin

Glandular- intracellular in origin

Muscular regions – intercellular in origin

o Nephridia with nephrostomes are open nephridia.

o Nephridia without nephrostomes are closed.

o The nephridia which open out through nephridiophore

are called exonephric nephridia.

o The nephridia which open into the gut are called

enteronephric nephridia.

oTypical described above are absent in Pheretima.

o In Pheretima three types of nephridia. They are:

Septal nephridia

Integumentary nephridia

Pharyngeal nephridia

SEPTAL NEPHRIDIA

oThe body of nephridium consists of short straight lobe, a long

spirally twisted loop with a narrow apical part and the

terminal duct.

o Septal nephridia are open nephridia because nephrostome

opens into coelom.

o These are enteronephric nephridia because waste products

are released into lumen of intestine

INTEGUMENTARY NEPHRIDIA

oThese are smallest nephridia hence called as

micronephridia(“ v” shaped structures )

o They are absent in the first two segments

o They are 200- 250 in each segment.

o These nephridia in the clitellar segments are called as

forests of nephridia.

o These are closed type because they donot have coelomic

funnel.

o These are exonephric nephridia they open out

independently through the nephridiopores on the outer

surface of body wall.

PHARYNGEAL NEPHRIDIA

o These nephridia occur in 4th, 5th and 6th

segments lying on either side of pharynx and

oesophagus as paired bunches

o Hence they are called Tufted nephridia.

o The pair of ducts from nephridial bunches of 6th

segment opens into buccal cavity while those 4th

and 5th segments opened into pharynx.

o These are closed nephridia.

oThese are enteronephric nephridia.

PHYSIOLOGY OF EXCRETION

oThe chief nitrogenous waste products are Ammonia,

Urea and Creatinine.

oThey are found in blood, coelomic fluid and

excretory fluid.

o Eleocytes are collected by septal nephridia from

coelomic fluid.

o Urine is hypotonic to both blood and coelomic fluid.

o Nephridia play an important in osmoregulation.

o These are described as ureotelic animals.

NERVOUS SYSTEM

o Nervous system consists of two parts

Central nervous system (CNS)

Peripheral nervous system (PNS)

o CNS consists of brain, a pair of circum-pharyngeal

connectives, a pair of sub-pharyngeal ganglia and a

double ventral ganglionated nerve cord.

o The brain or supra-pharyngeal ganglia lies in the

anterior part of 3rd segment dorsally in a groove

separating buccal chamber and pharnyx

o A pair of sub-pharyngeal ganglia lies in the 4th

segment . It is the centre of motor control

o A pair of circum-pharyngeal connectives, one on

either side of pharynx connects the brain and sub-

pharyngeal ganglia.

oThus a Nerve ring is formed around pharnyx.

o 4 giant axons are present on ventral nerve cord.

oIn each segment behind the 4th segment ,there is a

swelling of nerve cord formed by a pair of

segmental nephridia.

PERIPHERAL NERVOUS SYSTEM

o It includes nerves arising from the CNS and

innervating body parts.

o8 to 10 nerves are connected to the brain on each side.

they arise from prostomium and wall of buccal

chamber.

oThe circum –pharyngeal connectives gives off nerves

to the body wall of 1st segment and the wall of buccal

chamber.

oThe sub-pharyngeal ganglia supply nerves to the

different structures in the 2nd ,3rd and 4th segments.

The paired ganglia in each segment, from

5th segment to the last segment, give off

nerves to 3 pairs of nerves.

These branches supply to the gut wall,

bodywall and internal organs of respective

segment.

RECEPTORS

o The special nerve endings or cells whose function is

to receive stimuli are called Receptors.

o The different types of receptors are :

epidermal receptors- tactile(tango receptors)

Buccal receptors –Olfactory and Gustatory receptors.

Photo receptors-it is a single cell containing nucleus

and “ L” shaped optic organelle (lens0 consisting of a

hyaline substance.it is called PHAEOSOME.o Sensory hairs are absent

DIGESTIVE SYSTEM The digestive system of annelids comprises

of alimentary canal and his associated glands.

The alimentary canal consists of the following parts:-

Mouth-It is present at the anterior end. Buccal cavity-it has a folded walls. Pharynx-it is pear shaped and has salivaryglands and pharyngeal gland cells whichproduce mucus and proteolytic enzymes.

Esophagus-it occupies 4 segments(5th to 8th) and in the 8th segment it is modified to GIZZARD which helps in grinding the food materials, so it is known as GRINDING MILL.Stomach-it is also glandular and helps in the digestion of food by secreting proteolytic enzymes.Intestine-it is divided in to 3 regions based on the presence of TYPHLOSOLE.Pre-typhlosolar region –it has intestinal caeca which secrete amylase enzyme.

Typhlosolar region-this is the longest part of the intestine, it has typhlosole.Post-typhlosolar region-it is the terminal part of the intestine and it is also called has rectum.

PROCESS OF DIGESTIONDigestion is extracellular.Most of them are detrivores,some are carnivores and parasites.The undigested matter ejected by Earthworm is known as WORM CASTINGS.

CIRCULATORY SYSTEM

They have closed type of blood vascular system.In this respiratory pigment if present ,is dissolved in the blood plasma unlike in chordates where it occurs in certain blood corpuscles.The common respiratory pigment is haemoglobin like mammals. chlorocruorin(green) and haemoerythrin(pink or violet)are also met in some polychaetes.

BLOOD GLANDS

These are the red coloured follicular

bodies present in the 4th,5th,and 6th

segments.

These are red in colour owing to their

vascular nature.

REPRODUCTIVE SYSTEM

Earthworms are monoecious(bisexual and hermaphrodites).But cross fertilization occurs because of relative positions of male and female genital apertures and protandrous type of hermaphroditism.Male reproductive system contains testis,testis-sacs,seminal vesicles,spermiducal funnels, vasa deferentia,prostate glands,accessory glands,copulatory papillae and genital pores.

Female reproductive system consists of a pair of ovaries, a pair of oviducts and 4 pairs of spermatheca and female genital aperture in 14th segment.Cross fertilization occurs.For the formation of cocoon a membranous girdle is formed by the secretion of clitellar glands.Fertilization occurs after the deposition of cocoon.Development-cleavage is holoblastic (complete),determinate,spiral and unequal type.Leeches have internal fertilization.

        

What Are the Three Classes of Phylum Annelida?

There are many classes of Phylum Annelida. One major class is the Polychaeta, which are marine worms. Another class is Hirudinea, the class of leeches. Another class is Oligochaeta which are freshwater and terrestrial worms, like the common earthworm

Class POLYCHAETA          Polychaetes are mostly marine but some are freshwater or brackish water species. Commonly found from the intertidal zone to depths of over 5,000 m. The polychaetes are so named because of the numerous setae or chaetae they bear. They have about 8,000 species. Each body segment has a pair of locomotory organs called parapodia that bear many bristles called chetae, because of which polychaetes are sometimes referred to as bristle worms. Body size varies from 1 mm to about one metre. Polychaetes differ from other annelids in having a well differentiated head with sense organs, such as eyes, antennae and sensory palps and there is no clitellum on body.  Polychaetes are dioecious but have no permanent sex organs and no gonoducts. Some polychaetes are free swimming pelagic forms, some are active burrowers and crawlers, while others are sedentary, living in tubes or burrows and feeding on planktons and detritus. Based on their habitat they are divided into two subclasses:

1. Subclass ERRANTIA          They are free swimming or crawling worms in which body possesses numerous, identical segments and well-developed parapodia and setae. There is a definite head, with an eversible pharynx having jaws or teeth. The group includes swimming, crawling, burrowing and tube-dwelling members. Ex. Nereis; Polynoe; Aphrodite; Glycera; Eunice.

2. Subclass SEDENTARIA          They are tube dwelling worms that display a high degree of segmental differentiation. Parapodia are reduced, with or without modified setae. Prostomium without sensory structures but has tentacles and palps or other feeding structures. Teeth or jaws are generally absent. Ex. Arenicola; Terebella; Amphitrite; Serpula.

Class OLIGOCHAETA          They are worms without a distinct head and parapodia or suckers. There is a clitellum which produces a cocoon during breeding season. Pharynx is not eversible. They are hermaphrodite and there is no larval stage. There are about 10,000 species. Locomotion is by small setae and burrowing . Most of them are mud ingesting and detritus feeders but few are predaceous, e.g. Agriodrilus and Phagodrilus. There are two orders under class Oligochaeta.      

1. Order ARCHIOLIGOCHAETA (=TUBIFICIDA)            Freshwater forms with worm-like body and setae present in bundles. Eye spots may be present. Gizzard is poorly developed. Generally live in muddy tubes. Ex. Tubifex; Dero; Chaetogaster; Aelosoma; Nais; Stylaria; Enchytraeus.  2. Order NEOLIGOCHAETA (=HAPLOTAXIDA)            Commonly known as earthworms, they are terrestrial worms with well developed gizzard and without eye spots. Clitellum well developed and female genital opening on the 14th segment. Minute setae are used in peristaltic locomotion and hydraulic skeleton is used for burrowing in mud. Ex. Pheretima; Lumbricus; Eudrilus; Moniligaster; megascolides.

Class HIRUDINEA            They are ectoparasites of vertebrates having definite number of segments and annuli on each segment and without parapodia. Suckers are present for attachment and locomotion. They are hermaphrodite and deposit their eggs in cocoons. There are 4 orders as follows:            1. Order ACANTHOBDELLIDA, Ex. Acanthobdella.  Parasitic on salmons.            2. Order RHYNCHOBDELLIDA, Ex. Pontobdella. Aquatic leeches.            3. Order GNATHOBDELLIDA, Ex. Hirudo, Hirudinaria. Common cattle leeches.            4. Order PHARYNGOBDELLIDA, Ex. Erpobdella. Terrestrial predatory leeches. 

 

Class ARCHIANNELIDA            They are primitive marine worms, without distinct segmentation, parapodia and setae. There are 2 or 3 tentacles on prostomium. Anus is terminal. Larva is a typical trochophore. Ex. Polygordius; Protodrilus, Nerilla; Dinophilus.

HARMFUL EFFECTS

some of the members of the class

hirudinia are ectoparasites on

human beings and some animals

Some many contain harmful venom

BENIFITSEarthworms are used as a bait for fishing.

They are also used as food for fish in aquarium.

Many animals like frogs,lizards,birds feed on

the earthworms.

In unani(treatment of diseases with the help of

animals)medicine earthworms are used to treat

the human diseases such as stones in the urinary

bladderand gout.

Earthworms are also called as FRIENDS OF

FARMERS as they continuosly make burrows in

the soil and increas the fertility.

Earthworms are also used in making manures

such as vermicompost(solid),and vermiwash

(liquid)type of manures.

Vermicompost is known to contain 60%of

nitrogen,5.04%of phosphorus,and 0.8%of

proteins and micronutrients.