Pheretima: Habit and Habitat and External Features

Pheretima: Habit and Habitat and External Features !

Habit and Habitat of Earthworms:

The earthworms are burrowers. They are found in the soil rich in decaying organic matters usually in gardens, pastures, lawns, irrigated farm lands, near the banks of the ponds, lakes and rivers.

The do not prefer to live in sandy, clay and acid soils which are deficient in organic matters. They generally inhabit an upper layer of earth to a depth of about 30 to 45 cm and even they go down up to 3 meters or more during summer in search of moisture.

The rainy season is the most favourable time for the earthworms, during this season they live in the superficial soil of the earth and they are often seen crawling on the surface in abundance. The earthworms are nocturnal in habit; during night hours they come out of their burrows in search of food, reproduction and exploring fresh habitat.

They breed during the rainy season. They are cold-blooded or poikilothermal animals as their body temperature fluctuates with the fluctuation in the surrounding temperature.

The way of the excavation of their burrows is very interesting; as the worm eats on the soil, so it goes on eating the soil progressively downwards, the organic contents of the soil are digested and absorbed in the gut, while residual soil is discharged by its anus at the opening of the burrow as castings.

Thus, the presence of earthworms is inferred by their castings, The castings of Pheretima form little heaps of small round pellets lying separately near the burrows, while in Eutyphoeus they form large cylindrical structures of 5 cm in height. The natural life span of earthworms varies from three and a half years to ten years.

External Features of Pheretima:

Shape and Size:

Pheretima posthuma has long, elongated, cylindrical narrow body which is bilaterally symmetrical. The anterior end is tapering, while the posterior end is more or less blunt.

The dorsal surface of the body is marked by the presence of dark median line of dorsal blood vessel which runs throughout the length of the body just below the skin, while the ventral surface is marked by the presence of genital openings and papillae in the anterior part of the body.

Size of earthworm varies from species to species and individual to individual of the same species. A mature earthworm may attain the size up to 150 mm in length and 3-5 mm in width.

Colouration:

The earthworm is slimy to touch and glistening dark-brown in colour due to the presence of a pigment porphyrin in its body wall which protects the body from otherwise effect of the bright light. The dorsal surface of its body is darker than the ventral surface.

Segmentation:

The body of earthworm consists of about 100-120 small ring-like segments or somites or metameres. It exhibits true segmentation, i.e., the external segmentation corresponds with internal segmentation. In other words, the whole surface of the body is divided into many ring-like segments by a distinct series of annular grooves or furrows representing the external segmentation.

Similarly, the interior of the body is divided by inter-segmental septa or coelosepta into small chambers representing internal segmentation. However, all the segments of the body are alike except the first and the last segments.

Setae:

In all the segments of the body except the first, last and clitellum there is a ring of chitinous setae lying embedded in the middle of each segment which project backwardly. Each seta arises from a setigerous sac of the skin and is a pale-yellow curved S-shaped pointed rod-like with a swollen middle part called nodulus.

The setae help in locomotion by holding the earth since they are directed backwards. The movement of setae is controlled by special type of muscles. With the help of these special type of muscles the setae can be moved in any direction and extended or withdrawn at the will of the earthworm.

Clitellum or Cingulum:

In a mature worm, there is a prominent circular band of glandular tissue known as the clitellum which completely surrounds the segments from 14th to 16th forming a girdle around these segments. The glandular cells of clitellum secrete mucus, albumen, and material for forming cocoons which assist in fertilisation as the eggs are laid in them.

Due to the presence of clitellum the body of earthworm is divided into following regions:

(i) Preclitellar Region:

This is the anterior part of the body from segment 1st to 13th. The first segment is characteristically called peristomium (peri = around + stoma = mouth) which surrounds the mouth. At the anterior end of peristomium there is a small fleshy lobe called prostomium (pro = before + stoma = mouth).

The prostomium is not a segment and remains separated from the peristomium by a groove. In fact, the prostomium is an extension of the peristomium at its dorsal side and ventral to it is a mouth.

(ii) Clitellar Region:

This region is formed by segments 14th, 15th and 16th as described earlier.

(iii) Post-clitellar Region:

This region is represented by all the segments of the body after clitellum, i.e., it starts from 17th segment up to the last segment of the body.

External Apertures of Pheretima:

1. Mouth:

It is a crescentic aperture situated just below the prostomium on the ventral side surrounded by first segment of the body—the peristomium or buccal segment.

2. Anus:

It is the exit of the alimentary canal by which undigested wastes are removed. It is situated at the terminal end of the last body segment referred to as the anal segment.

3. Genital Openings:

Earthworm is hermaphrodite, so male and female generative apertures are found in the same individual.

(i) Male genital apertures:

A pair of male genital apertures are situated latero-ventrally in the 18th segment. These are crescentic openings through which the male reproductive bodies are discharged.

(ii) Female genital aperture:

It is a median aperture situated at the ventral side in the 14th segment through which the female reproductive bodies are discharged.

4. Spermathecal Pores:

There are four pairs of small ventrolateral spermathecal apertures lying inter-segmentally between the grooves of the segments 5/6, 6/7, 7/8 and 8/9.

5. Nephridiopores:

A large number of very minute nephridiopores are found situated throughout the body of earthworm except few anterior segments. These pores are the apertures of integumentary nephridia, through which metabolic wastes of the body are removed. These pores are found scattered all over the surface of the body.

6. Dorsal Pores:

These pores are situated along the mid-dorsal line in the inter-segmental grooves as minute openings and lead directly into the body cavity. The first dorsal pore lies in the furrow between segments 12th and 13th, while there is a pore in each of the succeeding inter-segmental furrows of the worm, except the last. Through these pores coelom communicates with the exterior.

Genital or Copulatory Papillae:

In the same line, with the male pores, on the ventral side of each of the 17th and 19th segments there is a pair of circular and raised papillae; these are the genital or copulatory papillae. Each genital papilla bears a cup-like depression at the top but not any aperture. During copulation, the genital papillae function as suckers.

Body Wall of Pheretima:

The body wall of earthworm is very thin, soft, shiny and elastic which consists of the following layers:

1. Cuticle:

It is a thin, elastic, non-cellular and iridescent layer covering the body externally.

It is made of two layers of collagenous proteins forming fibres and a polysaccharide with a small amount of gelatin. It is secreted by the columnar epithelial or supporting cells of the epidermis and remains perforated by many minute pores through which integumentary nephridia and epidermal glands open out. This layer is protective in function.

2. Epidermis:

The cuticle is followed by a single layer of tall columnar cells forming the epidermis.

The epidermis consists of the following four types of cells:

(i) Supporting cells:

These are long columnar cells constituting a major part of the epidermis and they have an oval nucleus nearly in their middle.

(ii) Gland cells:

These cells are situated between the supporting cells and are of two types

(a) Mucous gland cells:

The cells are club-shaped and are in large numbers. They secrete mucus which keeps the body moist. Thus, it prevents the animal from desiccation and helps in locomotion by lubricating the body. The mucus secreted by these cells cements the wall of the burrows to keep them lubricated and smooth. The mucous gland cells are also known as goblet cells and they open at the surface of the cuticle by minute pores.

(b) Albumen gland cells:

These cells are cylindrical, fewer in number and have uniformly distributed fine granules. These cells secrete albumen.

(iii) Basal cells:

These cells are small and undifferentiated which fill up the spaces between the inner ends of the other epidermal cells. The cells later differentiate to form the supporting and gland cells, hence, they may be called replacing cells.

(iv) Sensory cell:

These cells are cylindrical and lie in groups having sensory hair-like protoplasmic processes at their outer free ends. These cells are also known as epidermal receptor cells.

3. Muscles:

The muscles of the body wall are present just below the epidermis which are of two distinct layers—the outer circular muscle layer and inner longitudinal muscle layer.

The circular muscle layer is thin and in the form of a continuous layer around the body. This muscle layer contains many granules of pigment—porphyrin. The longitudinal muscle layer is much thicker, nearly two times than the circular muscle fibres and lies in separate longitudinal bundles.

These bundles are separated together by thin connective tissue septa. The contraction of circular muscles makes the body long and narrow, while that of the longitudinal muscles makes the body short and broad. The longitudinal muscle layer is further followed by a very thin strip of circular muscle fibres. The musculature of the body wall consists of smooth muscle fibres.

4. Coeiomic epithelium:

The musculature of the body wall is lined internally by a thin layer of coeiomic epithelium. It is a membranous layer consisting of thin flattened squamous cells of mesodermal origin. Since, this layer forms the outer boundary of coelom, hence, it is called parietal peritoneum or parietal layer of coeiomic epithelium.

Functions of the body wall:

The body wall of earthworm performs the following functions:

1. It forms an outer covering of the body to protect the internal more delicate organs from injury.

2. It provides definite shape to the body.

3. The mucus secreted by the mucous gland cells of epidermis keeps its body smooth and moist to assist in locomotion and respiration.

4. The mucus also keeps the burrow smooth and moist which assists in checking desiccation and cementing the wall of the burrow.

5. The sensory cells of the epidermis are the only receptors to receive stimuli.

6. The setae which are locomotory structures are found lodged in it.

7. The contraction and relaxation of the muscles of the body wall, assisted by setae, bring about locomotion in this animal.

8. The thin, soft and moist body wall helps in gaseous exchange which facilitates respiration.

9. The coelomic epithelium or peritoneal layer secretes the coelomic fluid.

10. The cuticle checks excess evaporation.

Setae, setal sac and associated structures:

As referred earlier, the setae are found in every segment of the body except the first, last and clitellar segments. In Pheretima nearly 80-120 setae are found embedded in the middle of each segment arranged in a ring. Such an arrangement of setae is called perichaetine. In Lumbricus, the setae occur in two pairs on either side of a segment. This is called lumbricine arrangement. Each seta is found lodged in a setal sac.

The seta is secreted by a single large epidermal cell situated at the bottom of the setal sac. The setae are composed of chitin and scleroprotein. The setae are pale-yellow, curved S-shaped pointed rod like structures with swollen middle part called nodulus. Two sets of muscles are found attached at the base of each setal sac the protractor and retractor muscles.

The protractor muscle radiates outwards from the base of the setal sac to join the circular muscles, while the retractor muscle runs inwards and joins the inner thin strip of circular muscle band situated just above the parietal peritoneum. These muscles serve to protrude and withdraw the setae respectively. The setae fall out of the body wall when worn out and are replaced by new ones repeatedly.

Coelom of Pheretima:

It is a large spacious cavity between the body wall and alimentary canal and encloses all the internal or visceral organs of the body, hence, it is also called perivisceral cavity. This cavity is lined by parietal epithelium on the outer side and visceral epithelium on the inner side.

Actually, this cavity of earthworm is a true coelom derived by the splitting of embryonic mesoderm, such a coelom is called schizocoel. The outer lining of coelom, lying below the body wall, is called parietal layer or somatic layer and that around the alimentary canal is called visceral or splanchnic layer. Many cells of the splanchnic layer containing yellow granules, are called chloragogen cells.

The spacious coelom of earthworm is divided into small compartments due to the development of inter-segmental septa which grow from the inter-segmental grooves of the body wall to the alimentary canal. Each septum is a sheet in interlacing muscle fibres and surrounded from both the sides by peritoneum.

However, the first four segments are without inter-segmental septa, hence, they have a continuous coleom. The first inter-segmental septum is located between 4th and 5th segments, it is thin and membranous. The next five septa are thick, muscular and obliquely placed between segments 5/6, 6/7, 7/8, 8/9 and 10/11.

These are cone- shaped. Contraction of these cone-like septa increases the pressure on the coeiomic fluid by which the anterior segments become turgid and are used for digging the burrow.

Inter-segmental septum between 9/10 is absent. The remaining septa from 11/12 to the posterior end are thin, membranous and transverse; the first three of them (11/12, 12/13 and 13/14) are complete with no apertures, thus, isolating their coeiomic chambers.

The remaining septa from 14/15 to the posterior end are perforated by many minute apertures, these apertures are surrounded by sphincters of un-striped muscles. Such septa are characteristic of Pheretima. With these perforations, the coeiomic chambers of these segments remain in communication. The coelom is filled with a coeiomic fluid.

The coeiomic fluid of earthworm is milky white and alkaline. It consists of colourless watery matrix the plasma and a large number of coeiomic corpuscles.

The corpuscles are differentiated into four types:

(i) Granulocytes:

These are numerous, large-sized and spherical with a concavity having small nucleus. They have many granules of fat droplets. These corpuscles form pseudopodia-like folds to act as phagocytes. These are probably nutritive and help in removing the harmful bacteria, foreign particles and other microorganisms that happen to be in the coeiomic fluid by phagocytosis. These are also called eleocytes.

(ii) Amoebocytes:

These are also numerous but much smaller having many pseudopodia-like processes. They look like stars and nucleated. These are phagocytic, they take up excretory and foreign matter and act as defence against parasites.

(iii) Mucocytes:

These cells are elongated, vase-shaped with an expanded end like a fan and the other end being narrow. The narrow end bears nucleus. Their function is not definitely known.

(iv) Leucocytes:

These are small, flattened disc-shaped and nucleated cells. They are fewer in number, nearly 10 per cent of the corpuscles. They have clear cytoplasm and their functions are still uncertain.

The coelomic fluid flows from anterior to posterior on the ventral side and from posterior to anterior on the dorsal side.

Functions of Coelomic Fluid:

The coelomic fluid performs following functions:

1. It oozes out through the dorsal pores and keeps the body most which facilitates respiration.

2. It kills the bacteria of the soil which come on the body surface of the worm.

3. It helps in removing the excretory wastes from the body.

4. It helps in the transportation of materials from one part of the body to the other.

5. It helps in destroying harmful bacteria, micro-organisms and foreign materials.

6. It also helps in fixing the setae into the ground during locomotion. This is achieved by making certain segments turgid and stiff by restricting coelomic fluid in them by closing sphincter apertures of the septa between them.

7. The luminosity produced by some earthworms is due to the coelomic fluid.

Lymph Glands of Pheretima:

These are small paired glandular structures present in the coelom on the sides of the dorsal vessel from 26th segment up to the last. They also contain phagocytes which ingest foreign materials and digest them.

Locomotion of Pheretima:

No specialised locomotory organs are found in earthworms, even then these are very active and they crawl rapidly when out of burrows. The process of locomotion is, in fact, a cumulative effect of contraction and relaxation of both the muscle layers of body wall aided by setae and the hydrostatic pressure created by the coelomic fluid.

The forward locomotion is brought about by an increase in the hydrostatic pressure of the anterior segments of the body (usually first nine segments) and at the same time contraction of circular muscle layer begins at the anterior end and passes backwards. This results the anterior region to extend forward and at the same time making it thinner in diameter.

The extending and thinning passes backwards as a wave of contraction at the rate of 2 to 3 cm per second; by this means the body of the worm is pushed forward. The anterior end now grips the substratum and the setae act as hooks by their posteriorly directed points.

After the wave of contraction of circular muscles has passed down the front half of the worm, the circular muscles relax and the longitudinal muscles of the body wall of the anterior segments contract in a wave beginning from the anterior end, this shortens and thickens the anterior end, causing the posterior body of the worm to be dragged forward.

At this stage the segments in a state of longitudinal contraction do not move but remain anchored to the ground by the extended setae. The contraction of longitudinal muscles passes backwards like a wave. Again a wave of contraction of circular muscle starts from the anterior end making it thinner and extended forward.

This wave starts before the first one has reached to the posterior end. Thus, locomotion is brought about by alternate contractions of circular and longitudinal muscles causing wave of thinning and thickening to pass backward. This involves partly a pushing of the anterior end and partly a pulling of the posterior end, the setae playing only an accessory role.

The worm moves at the rate of 25 cm per minute. The nervous system coordinates the activities of circular and longitudinal muscles so the contraction of any layer brings about relaxation of the other muscle layer.

The earthworms can move backward also, as they usually do during their withdrawal from the burrow and also during excavating it.

The backward movement is brought about by reversing the direction of setae. The coelomic fluid serves as a hydraulic skeleton because a decrease in its pressure results in the relaxation of muscles. The earthworms can move on a smooth and hard surface like glass by using mucus for adhesion as the setae cannot anchor the substratum.

Respiratory System of Pheretima:

Although earthworm is a terrestrial animal but its mode of respiration is more like that of simple aquatic animals. The earthworm is devoid of special respiratory organ.

The exchange of gases takes place through the body wall which is thin, semitransparent and richly supplied with blood capillaries. The tegumental glands and coelomic fluid make the surface of the skin moist with their secretions to assist in gaseous exchange. It has been observed that if its skin dries the worm dies of suffocation, i.e., asphyxia.

As mentioned earlier the blood of earthworm contains a respiratory pigment—haemoglobin in a dissolved state in its plasma. The epidermis of the body wall acts as a permeable membrane through which the atmospheric oxygen diffuses in its capillaries and combines with haemoglobin to form oxyhaemoglobin.

The oxyhaemoglobin is circulated by the blood into the tissues where oxygen tension is very less and CO₂ tension is high. The oxyhaemoglobin breaks up to release oxygen to the tissues and haemoglobin in a reduced state. This oxygen is utilised in the tissues for the oxidation of foodstuffs to release energy needed for performing the vital activities of life and carbon dioxide.

At the same time, CO₂ from the tissues diffuses into the blood due to its high tension.

The CO₂ is carried by the blood generally in a dissolved condition and when it reaches to the epidermal capillaries the CO₂ from the blood diffuses out in the atmosphere due to low CO₂ tension at this level and again oxygen diffuses in the blood due to its high tension and combines with haemoglobin to form oxyhaemoglobin once again. Thus, respiration is affected in earthworm.

Nervous System of Pheretima:

The nervous system of Pheretima is well developed, concentrated and consists of the central, peripheral and sympathetic or autonomic divisions.

1. Central Nervous System:

It consists of the nerve ring or brain ring and the ventral nerve cord.

Nerve Ring:

A pair of cerebral or supra- pharyngeal ganglia is found fused together to form the so-called brain which lies dorsally in the 3rd segment in the groove between the buccal chamber and the pharynx.

From the brain a pair of circumpharyngeal connective arise, one on each side, they encircle the pharynx and meet ventrally in the 4th segment by a pair of fused sub- pharyngeal ganglia. Thus, a nerve collar is formed around the pharynx which is called the nerve ring or brain ring.

Nerve Cord:

A nerve cord originates from the sub-pharyngeal ganglia and runs on the mid-ventral side beneath the ventral vessel but above sub-neural vessel extending up to the posterior end of the body. The nerve cord is double and is made of two longitudinal cords fused together.

In each segment from 5th to the last the nerve cord has a pair of ganglia fused together as swellings representing the segmental ganglia. The nerve cells are distributed all over the nerve cord and are not confined to ganglionic swellings.

Structurally, the double nerve cords of Pheretima are solid and enclosed in a common sheath called perineurium consisting of three layers— the outer peritoneum, middle longitudinal muscle layer and inner thick fibrous layer of epineurium. The nerve cords are solid consisting of the nerve cells and the fibres. Both the nerve cords are separated internally by a vertical epineurium.

The ventro-lateral regions of nerve cord contain bipolar and tripolar nerve cells all along the length, and the middle region has many nerve fibres with supporting neuroglia. Running longitudinally in the upper part of the nerve cord are four giant nerve fibres surrounded by epineurium, they are all tubular and filled with homogeneous plasma-like matter.

These are responsible for rapid conduction of impulses throughout the nerve cord. However, in the region of segmental ganglia the partition between the two nerve cords is absent.

2. Peripheral Nervous System:

From the brain 8 to 10 pairs of nerves arise which supply to the prostomium, buccal cavity and pharynx. Two pairs of nerves arise from the circumpharyngeal connectives which supply to the first segment and buccal cavity. From the sub pharyngeal ganglia three pairs of nerves arise to supply 2nd, 3rd and 4th segments.

The nerve cord and ganglia of each segment give out three pairs of nerves which supply the various parts of the segment in which the ganglia are situated. Thus, the nerves originating from the central nervous system to supply the different parts of the body constitute the peripheral nervous system.

3. Sympathetic Nervous System:

It includes the nerve plexuses situated in the wall of the alimentary canal and some other internal organs. Such plexuses are connected with the peri­pharyngeal connectives by fine nervules and help in coordinating the functions of the related organs.

Working of the Nervous System:

The nervous system of Pheretima, like those of higher animals, has both sensory and motor neurons. The nerves of the nerve cord, therefore, have both sensory and motor fibres, i.e., mixed. The stimuli from the skin are transmitted by sensory fibres to the nerve cord and the stimulus is transferred either to an association neuron first, or it may go to the muscles (effectors) which cause them to contract.

This circuit of stimuli or impulses forms a simple reflex arc. The movements of circular and longitudinal muscles are coordinated, so that contraction of one brings about relaxation of the other. The impulses are conducted more rapidly in the giant fibres resulting into sudden contraction of the whole body of earthworm.

Sense Organs of Pheretima:

Pheretima reacts to a number of external stimuli with the help of three types of sense organs or receptors epidermal receptors, buccal receptors and photoreceptors.

1. Epidermal Receptors:

Epidermal receptor (Pig. 66.28A) consists of an ovoid group of tall cells in the epidermis which cause an elevation of the cuticle. These cells are separated from each other by spaces. Each cell has a nucleus near the middle and it ends above in slender hair-like processes which penetrate the cuticle and project beyond it.

The lower ends of these cells are supplied with nerve fibres. The epidermal receptors are found all over the epidermis but are more numerous in the lateral and ventral regions. They are tactile in function, i.e., tango receptors and also perceive thermal and chemical stimuli. Hence, earthworms are very sensitive to touch and to vibrations transmitted through solid objects, though they cannot hear at all.

2. Buccal Receptors:

These are found in the epithelium of buccal cavity in large numbers, they consist of groups of tall cells which project beyond the epithelial cells. These cells (Fig. 66.28B) have sensory hair-like processes and their nuclei lie below the middle part of the cells.

These receptors serve to smell, i.e., olfactoreceptors and taste food, i.e., gustatoreceptors. These cells can distinguish between the tastes of different vegetable foods but their sense of smell is poorly developed, though earthworms can smell different types of decaying leaves used as food.

3. Photoreceptors:

A photoreceptor (Fig. 66.28C) is a single ovoid cell in the inner part of epidermis, it has a nucleus and cytoplasm with a network, and an optic cell (phaosome or lens) generally of curved shape and made of a hyaline substance. One or two nerve fibres enter this optic cell.

Photoreceptors are found mostly on the prostomium and first segment, they occur in lesser numbers in other segments including the last. They are not found on the ventral surface.

Photoreceptors are sensitive to light and also called little eyes or ocelli. Actually the network of a photoreceptor is formed by branching and re-branching of the nerve fibre entering into it and characteristically called retinella. The earthworms show a negative response to even a very weak light, hence, they retreat into their burrows during day and come out at night, i.e., they are nocturnal in habit.

Development of Earthworm:

The cocoon contains fertilised egg and albuminous substance secreted by the glands of the clitellum for nourishment of the embryo.

Development (Fig. 66.38) occurs in the cocoon with no larval stage. The zygote undergoes holoblastic and a modified spiral cleavage resulting into a hollow ball of cells, the blastula, enclosed into a vitelline membrane. The lower cells of blastula are endodermal and those of upper cells are ectodermal.

Gastrulation occurs by invagination of endodermal cells into ectodermal cells to form a cylindrical gastrula with an archenteron cavity and a blastopore which narrows to become the mouth. Two large mesoblast cells are differentiated early and they give rise to the mesoderm band.

The embryo escapes from the vitelline membrane and begins to feed on albuminous substance in the cocoon. The embryo elongates, the mesoderm band extends between the ectoderm and endoderm and it gets divided into segments. Each segment of the mesoderm acquires a coeiomic cavity.

The ectoderm cells form thick rows behind a large cell known as teloblast, the innermost row forms the nerve cord, and the next two rows of ectoderm give rise to nephridia. The mouth and anus open into the endodermal enteron to form the alimentary canal. A young earthworm is, thus, formed which escapes from the cocoon to lead an independent life.