Here is a term paper on ‘Planaria (Dugesia)’. Find paragraphs, long and short term papers on ‘Planaria (Dugesia)’ especially written for college and medical students.
Term Paper on Planaria (Dugesia)
Term Paper Contents:
- Term Paper on the Habits and Habitat of Planaria
- Term Paper on the Morphology and Structure of Planaria
- Term Paper on the Locomotion of Planaria
- Term Paper on the Digestive System of Planaria
- Term Paper on the Histology of Alimentary Canal of Planaria
- Term Paper on the Excretory System of Planaria
- Term Paper on the Respiration and Nervous System of Planaria
- Term Paper on the Sense Organs of Planaria
- Term Paper on the Reproduction in Planaria
- Term Paper on the Development and Regeneration of Planaria
Term Paper # 1. Habits and Habitat of Planaria:
ADVERTISEMENTS:
Planaria is fresh water terbullarian. It belongs to class Turbellaria. Usually Turbellarians one free living un-segmented flatworms. These remain clothed with a cellular epidermis, which is usually ciliated, sometimes only in parts. These are primarily aquatic and great majority are marine. There are also some pelagic species but majority of them are bottom dwellers which live in mud, sand, under-stones, shells or on sea weeds. The present description belongs to fresh water Dugesia which is formerly called Planaria.
Dugesia is a fresh water form commonly found in ponds, lakes, streams and shallow rivers on the underside of leaves, logs, debris and rocks. They are gregarious and live in groups. As they are small, flattened and blend perfectly with the surrounding leaves or rocks, they can be traced only when moving. It is world-wide in distribution.
Planarians can be collected by baiting the shallow streams with pieces of beef or liver between the rocks or logs and other debris. Planarians stick to them and may be collected. They can now be stored in jars containing pond water. Small pieces of stones can be introduced to provide hiding space and small pieces of algae or meat or small crustaceans will form the food. The water can be changed after three or four days.
Term Paper # 2. Morphology and Structure of Planaria:
ADVERTISEMENTS:
Morphology:
Shape, Size and Colour:
Planaria is elongated, leaf-like flattened having bilateral symmetry. Its body wall is tin and transparent. It measures about 1.5 cm in length. Its body colour is greyish, brownish or blackish having spots or streaks. Its dorsal is darker than the ventral side.
Structure:
ADVERTISEMENTS:
A head is present at the anterior end of the body. Therefore, two projections called auricles or head lobes are present in the head. The head bears a pair of black eyes which are cup- shaped. The ventral surface of the body rose like an arch.
The head is separated from the body by constriction. A mouth is present on the median line of the ventral surface a little behind the middle of the body. It opens behind into the pharynx which is covered by a pharyngeal sheath. The pharynx can be protruded out the mouth in the form of the proboscis.
A genital aperture or gonopore is situated a little behind the mouth. Many excretory pores or nephridiopores are found on the dorso lateral surface of the body. An adhesive zone, on which numerous pores of adhesive organs are situated, is found along the ciliated edges of the ventral surface. These glands secrete mucous which helps the animal in sticking to the substratum.
Body Wall:
The body wall of Planaria hash an outer epidermis, inner muscle layer and in between the two is the basement membrane. Rhabdites are arranged at right angles to the body surface. They are hyaline elliptical structures secreted by rhabditogen cells.
When they are expelled out they swell on contact with water forming a sticky froth around the body. This may be protective, help in gliding or even in capture of food. Some workers like Reisinger and Kelbetz (1964) suggest that the rhabdites and nematocysts are evolutionary related.
Below the epidermis are granules and rods of pigments. The gland cells are unicellular, some occur in the epidermis but most of them are in the mesenchyme. They have long necks opening on the surface and secrete mucous. Below the epidermis is a thin elastic basement membrane at which the epidermal cells rest. It maintains the general form of the body and provides surface for the attachment of underlying muscles.
(a) Epidermis:
It is formed of a single layer of cuboidal or columnar cells having basal nuclei. The epidermal cells of the ventral side are richly ciliated. In the epidermal cells are present rod-shaped rhabdites which are numerous in the dorsal epidermal cells. They are not found in the epidermal cells of the adhesive zone and in the sensory cells. Mucous secretory glands cells lies between the epidermal cells. Sensory cells are also found scattered in the epidermal cells.
ADVERTISEMENTS:
Two types of glands are found in the epidermis:
(i) Mucous Glands:
They are unicellular tubular glands which are formed by the transformation of epidermal cells but lie in the mesenchyme.
(ii) Adhesive Glands or Eosinophilous Glands:
They are epidermal glands found in the mesenchyme. They are found in groups and situated in the adhesive zone on the ventral surface.
(b) Basement Membrane:
It is thin and structure-less. Over it lie the epidermal cells and beneath it are present muscles which are attached with it.
(c) Muscle Layer:
It is formed by elongated contractile muscle cells.
These muscle cells form three layers beneath the basement membrane:
(i) Outer circular muscle layer,
(ii) Middle diagonal muscle layer, and
(iii) Inner longitudinal muscle layer.
The last layer is more developed on the ventral side. Dorso-ventral muscles are also found extending in between the dorsal and ventral surfaces. Pigment cells are present beneath the circular muscles.
(d) Mesenchyme or Parenchyma:
It is a connective tissue found beneath the muscle layer in between the different organs of the body. Its cells are vacuolated, syncytial and stellate, i.e. cells having irregular processes. These cells form a loose network. The intercellular spaces are filled with fluid and possess amoeboid cells which help in the regeneration of injured tissues. The mesenchyme also contains rhabdites forming gland cells and mucous secreting gland cells.
Term Paper # 3. Locomotion of Planaria:
Planaria is aquatic but it does not swim in water. It always moves in contact with some substratum or on the underside of water surface film.
In Dugesia, the locomotion takes place in the following two ways:
(i) Gliding movement
(ii) Crawling
(i) Gliding Movement:
It occurs with the help of cilia present on the ventral surface of the body. First mucous is secreted over the substratum, then the cilia beat backwards in the mucous due to which it moves forward with the head raised upward.
(ii) Crawling:
Planaria crawls forward over the substratum by means of contractions of circular and longitudinal muscles of the body. Due to the contraction of circular muscles, the body becomes narrow and elongated. Its head adheres to the substratum with the help of the muscous. Now the longitudinal muscles contract, the body becomes short in length and pulled forward.
Sometimes transverse waves of contraction sweep the length of the body causing its undulations in the vertical plane and enabling the Dugesia to crawl forward. Differential action of local muscle groups produces turning or twisting movements.
Term Paper # 4. Digestive System of Planaria:
The digestive system includes the mouth, pharynx and intestine which end blindly in the mesenchyme. Anus is absent.
(i) Mouth:
The mouth is an oval or rounded aperture situated mid ventrally a little behind the middle of the body. It is surrounded by radial and circular muscle fibres from the sub-epidermal musculature. The muscle fibres regulate the opening or closing of mouth. It leads into a large cylindrical elongated chamber, the pharyngeal pouch.
(ii) Pharynx:
Mouth leads into a specious pharyngeal pouch which is surrounded by pharyngeal sheath. Within the pharyngeal pouch is present a thick walled, muscular cylindrical pharynx. It is attached with the anterior end of the pharyngeal sheath. The opening of the pharynx lies close to the mouth. During feeding pharynx can be everted through the mouth in the form of a proboscis.
(iii) Intestine:
The pharynx posteriorly opens into the intestine which is divided into three branches, one median branch extending towards the head and two lateral branches which run towards the posterior side. Each branch of the intestine gives out numerous lateral diverticula which are also branched. The three branches of the intestine and their diverticula terminate blindly in the mesenchyme.
Term Paper # 5. Histology of Alimentary Canal of Planaria:
The pharyngeal wall shows a complicated structure. It consists of nine layers from the surface to the lumen, epithelial cells, longitudinal muscle layer, circular muscle layer, outer gland cells, nerve plexuses, inner gland cells, longitudinal muscle layer, circular muscle layer and an endodermal epithelial living.
The intestine is simple and thin consists of a single layered epithelium, the gastrodermis. The gastrodermal cells are of two types – phagocytic cells and granular cells.
Food and Feeding:
Planaria is carnivorous and feeds upon small crustaceans, insect larvae and also dead or living large animals. The animal first moves over its prey and then presses it in between the substratum and ventral body wall. It also secretes mucous over the prey. Now the pharynx is everted to ingest the prey. Smaller prey is sucked as such but the larger prey is first broken into pieces by the pumping movements of the pharynx. During this time the digestive juices are also secreted over the prey.
Digestion and Egestion:
The food is digested in the intestine and also in the food vacuoles of amoeboid cells present in the intestinal wall. Hence digestion is extracellular as well as intracellular. The digested food diffuses through the walls of the intestine into the surrounding mesenchyme. The digested food is supplied to all parts of the body by the mesenchyme and the highly branched intestine. The reserve food in the form of fat and protein globules is stored in the epithelial cells of intestine. The undigested food is expelled out through the mouth.
Term Paper # 6. Excretory System of Planaria:
The excretory system or protonephridial system of Dugesia consists of a pair of longitudinal excretory canals running on each side of the body throughout the length. They are highly inter-coiled and open to the dorsal surface by several minute pores called nephridiopores.
The two canals are united anteriorly in front of the eyes by a transverse vessel. Each trunk or canal divides into a number of branches which further divides into very fine capillaries which end in flame cells.
There are a large number of flame cells occurring along the entire body of the animal. Each flame cell is of considerable size and gives off a number of amoeboid and branched protoplasmic processes into the surrounding mesenchyme. In the centre of the cell is a large cavity and the cytoplasm is confined to the periphery.
This cavity is continued into the capillary. The cell has a rounded or oval nucleus of a few excretory globules and vacuoles. In the space of the flame cell are situated numerous long, hair-like flagella or cilia, each arising from the basal granule, situated in the cytoplasm. In the living animal, there flagella vibrate giving the appearance of a flickering candle flame, hence the name flame cell.
Physiology:
Excretory substances absorbed from the surrounding mesenchyme are passed into the cavity of the flame cells. The movement of flagella drives the liquid excretory wastes into the longitudinal excretory duct through capillary ducts etc. The cilia of these ducts drive the fluid toward the nephridiopores. Besides the flame cells, certain glandular cells, the paranephrocytes or arthrocytes are found in close association with the excretory units. They also help in removing the excretory products.
Term Paper # 7. Respiration and Nervous System of Planaria:
Respiration:
There are no respiration organs in Planaria. The oxygen diffuses into the body through the general body surface and the carbon dioxide diffuses outside in the same manner.
Nervous System:
The brain in Dugesia is situated in head region of the body. It is bilobed nervous structure and composed of two cerebral ganglions. It gives off nerve fibres to apex and auricles. The ventral nerve cords are given off from posterior end of the brain and extend along the lateral sides to the posterior end of the body. Each nerve cord contains many ganglions from which numerous nerve fibres are given off to be lateral sides of the body. The two nerve cords are joined with each other by transverse commissural bands.
Besides the central nervous system, a sub epidermal plexus is present below the epidermis and a sub-muscular plexus below the muscle layer in the mesenchyme.
Term Paper # 8. Sense Organs of Planaria:
The following types of sense organs are found in Dugesia:
1. Chemoreceptors.
2. Auricular organs
3. Eyes of Photoreceptors
4. Tangoreceptors.
1. Chemoreceptors:
The anterior end of the body possesses several pits and grooves. Their epidermal cells are ciliated and provided with sensory nerves. These cells are sensitive to chemicals and detect the food in the water current.
2. Auricular Organs:
A whitish groove called the auricular organs is present on each side of the head. These two grooves or auricles are ciliated and innervated. They serve as organs of chemical sense for smelling and tasting. In addition, several grooves and pit-like depressions are situated along the anterior end of the body.
The grooves are lined by non-ciliated and the pits by ciliated and richly innervated cells. These sensitive cells lining the grooves and pit are chemoreceptors and are sensitive to chemicals. They help in the selection of food and orienting the body towards the food.
3. Eyes of Photoreceptors:
These are a pair of round, dark spots on the dorsal surface of head. Each eye has a pigmented cup with its open mouth facing laterally forward. The cup is lined with a layer of pigmented retinal cells which form the retina. Numerous bipolar sensory nerve cells or photosensitive cells project into the hollow of the cup.
This type of eye is known an inverse eye in which the light first passes through the photosensitive cells and then reaches the retina. These sensitive nerve cells have expanded and striated inner ends while the outer ends joint the brain. There is no lenses hence an image is not formed. It can simply perceive the difference between light and dark. The animal is negatively phototactic and is most active at night.
4. Tangoreceptors:
These cells are present on the ventral surface especially at the anterior end and the sensitive to touch. Their sensory processes project above the epidermis.
Term Paper # 9. Reproduction in Planaria:
Planaria reproduces asexually as well as sexually.
1. Asexual Reproduction:
It occurs by transverse binary fission. A constriction appears behind the pharynx which gradually deepens and finally divides the body into two parts. Each part regenerates the required structures which the lacking in each part.
2. Sexual Reproduction:
Reproductive organ are temporarily formed during breeding season. After reproduction, these organs degenerate and the animal changes to asexual strain which reproduces by transverse fission till the summer of next year when it again reproduces sexually. The animal is hermaphrodite as both male and female organs are present in the same individual. Both male and female gonoducts open into a common genital atrium which opens to the exterior by common gonopore situated ventrally behind the mouth.
A. Male Reproductive Organs:
Testes are numerous small, rounded bodies situated on the lateral sides of the body. Each testis opens into a fine vas deferens or sperm ductile. These vas efferens open into the vas deferens or sperm duct of that side. These are two in number, one on either side and each receive a number of vasa efferentia from the testes of its own side.
From each vas deferens arises a dilated seminal vesicle in which sperms are stored. Both the seminal vesicles running posteriorly unite in the middle line to form a medial duct or canal. It runs through a protrusible muscular organ, the penis which opens into the genital atrium.
The proximal part of the penis is called the penis bulb which is muscular and glandular, and it is surrounded by numerous unicellular prostate glands which open into the penis bulb. The distal conical part of the penis is penis papilla which opens into the genital atrium. Penis protrudes through the gonopore at the time of copulation. Sperms are long provided with two flagella.
B. Female Reproductive Organs:
There are two rounded ovaries situated at the anterior end behind the eyes. Each ovary is connected with an elongated oviduct ovo-vitelline duct which runs parallel and dorsal to the nerve cord of that side. The two oviducts unite posteriorly to form a short median vagina or common ovo-vitelline duct.
It opens into the genital atrium. In the genital atrium also opens an elongated club-shaped uterus or seminal receptacle. During copulation it receives sperms from the other animal. Into the genital atrium also open a small muscular sac of unknown function. The genital atrium also receives the opening of cement glands which surround it. Each oviduct all along its length receives numerous branched vitelline glands.
Copulation and Fertilization:
Cross-fertilization is the rule as self-fertilization does not occur. During copulation, two individuals come in contact temporarily along the ventral surface with their genital apertures opposite each other. The penis papilla of each dilates and protrudes through its gonopore.
It reaches through the genital atrium into the bursa copulatrix of the other individual. Thus mutual insemination takes place and the sperms are discharged into the copulatory sac. Now the two worms separate. The sperms thus received are stored temporarily in the seminal receptacles. The secretion of prostate glands keeps the sperms active. The sperms now migrate into the oviduct and reach their proximal end where they fertilize the eggs when they come out of the ovaries.
The fertilized eggs or zygotes pass down the oviduct and get mingled with yolk cell secreted from yolk glands. The eggs and yolk cells collect in the genital atrium and get surround within a capsule cocoon formed by yolk cells.
The capsule is spherical and is provided with an adhesive stalk secreted by the cement glands. The capsule is laid on the under-surface of some submerged object like aquatic weed or stone to which they adhere by means of adhesive stalk. One animal copulates several times during the breeding season and lays a cocoon every few days.
Term Paper # 10. Development and Regeneration of Planaria:
Development:
Soon after the capsule is laid, the egg starts developing which is completed in two or three weeks. The development is direct and the cleavage is spiral and determinate. A young one is developed from each zygote and the yolk cells provide food to the developing embryo. When fully formed the egg capsule ruptures along a definite line and the young animal comes out of it.
Regeneration:
Planaria has great power of regeneration. If the animal is cut across two or more parts, each part regenerates into a complete and normal individual. When the anterior end of the animal is cut along the length into two or more parts, each part develops into a new head so as to form a many headed monster or heteromorphy.
If they body is cut into many pieces it is seen that a piece from the middle always regenerates a head towards its anterior side and tail towards the posterior side thus each piece maintains it original polarity. It is side that in Planaria, the metabolic activity and hence capacity for regeneration is the greatest at the anterior and gradually decreases posteriorly.
If a sexually mature individual is cut into two between the pharynx and the copulatory apparatus, then the reproductive organs degenerate and each piece will regenerate into an asexual form. Previously, it was considered that the interstitial cells are responsible for regeneration but it has been shown now that when the planarian is cut, the free cell from the mesenchyme called new-blasts migrates to cut surface and produce the last parts.
Regeneration is brought about by the free formative cells or neoblasts of the mesenchyme. These cells migrate to the cut surface and by repeated division produce new tissues. Regeneration is always not perfect. If a sexually mature Dugesia is cut transversely between its pharynx and copulatory apparatus, its reproductive organs degenerate and fail to regenerate, but each half grows into asexual animal.