In this article we will discuss about Dugesia:- 1. Habit and Habitat of Dugesia 2. Structures of Dugesia 3. Body Wall 4. Locomotion 5. Digestive System 6. Respiratory System 7. Excretory System 8. Nervous System 9. Reproductive System 10. Regeneration.
Contents:
- Habit and Habitat of Dugesia
- Structures of Dugesia
- Body Wall of Dugesia
- Locomotion of Dugesia
- Digestive System of Dugesia
- Respiratory System of Dugesia
- Excretory System of Dugesia
- Nervous System of Dugesia
- Reproductive System of Dugesia
- Regeneration in Dugesia
1. Habit and Habitat of Dugesia:
Dugesia is a member of the class Turbellaria and is free-living. Planarians are inhabitants of cool, clear and permanent water and cling to the under-surfaces of leaves, logs, rocks sub- merged in streams, ponds and lakes.
They live in damp surroundings as their bodies are not protected against dessication. They always avoid light and rest on the under-surfaces of objects during daytime either singly or in groups. After dark they become active. The distribution of Dugesia is world-wide.
2. Structures of Dugesia:
A full grown Dugesia is under 50 mm in length. Body is flattened, thin, leaf-like and oval in outline (Fig. 14.2). They exhibit bilateral symmetry and polarity is well-marked. There is no well-formed “Head” but a definite anterior end with organs of special sense is always directed forward in the direction of movement and it is broader than the other end, i.e., the posterior.
The body surface next to the substratum during locomotion is the ventral side and the uppermost surface is dorsal side. The anterior region is triangular with small lateral projections and bears a pair of black eye spots on the mid-dorsal line. Auricular organs are lodged in these projections.
The mouth is situated in the mid-ventral surface. Sexually mature worms have a single genital aperture situated below the mouth. Excretory apertures are minute and situated on the dorsal surface. The colour, though variable in different species, is usually grey, red, brown or black.
3. Body Wall of Dugesia:
The outer covering of the body is epidermis which is ciliated on the ventral surface and contains many scattered sensory and gland cells (Fig. 14.3). In the epidermis there are many rod-shaped hyaline structures, called rhabdites (Fig. 14.4). These are produced by epidermal gland cells in the mesenchyme below the epidermis. They are found on the dorsal surface rather than ventral surface.
Another, small, numerous related structures are also found in the epidermis, called rhabdoids. The rhabdites, when discharged come in contact with water swell, stick together and form a froth. The exact role of rhabdites is not known and probably they release a thick mucus coat that forms a protective and adhesive sheath around the body. It is also assumed that they help in capturing food particles or in desiccation.
The epidermis of the ventral surface contains a large number of paired secretory glands, called duo glands which open to the exterior. Out of two, one is called adhesive gland which produces viscous glue substance by which the animal is attached to the substratum and the other is releaser gland which secretes a chemical substance that breaks the attachment to the substrate.
Just beneath the epidermis there is a thin, collagenous fibrous sheet, called basal membrane which serves as a partition between epidermis and underlying muscle layers. The basal membrane is stratified and bears pigments below. Beneath the basal membrane the muscle fibres are arranged in two layers.
The fibres of the outer layer are arranged in a circular manner around the body and are called circular muscles while the inner fibres lie parallel to the longitudinal axis of the body and are called longitudinal muscles.
In addition, a third layer of oblique fibres arranged in vertical fashion is also encountered. The longitudinal muscles of the ventral side are more strongly developed than their counterparts on the dorsal side.
Lying between the gut and muscular layers of the body wall is the mesenchyme or called parenchyma tissue. These are loose connective-tissue cells, parts of which remain in formative state and can metamorphose into specialised cells.
They help in regenerating lost or damaged body parts. They also undertake vascular function by conducting food and other metabolic products from one part of the body to another part. Embedded in the parenchyma there are many gland cells.
4. Locomotion of Dugesia:
Dugesia lives in water, but it does not swim. Locomotion is performed usually by gliding (Fig. 14.5A). During gliding, the anterior end remains forward and slightly raised. The backward stroke of the cilia on the ventral surface helps the animal to move forward over a slime track. The slime track is produced by the secretion of glands in the body wall.
Sometimes the animal also crawls. This is performed by muscular movements. Contraction of circular and dorso-ventral or oblique muscles causes elongation of the body. The anterior end is then fixed firmly on the substratum by mucus and the posterior part is drawn up by the contraction of longitudinal muscles.
Turning and twisting movements are also shown by planarians. This is done by different action of total muscle groups.
5. Digestive System of Dugesia:
The digestive system consists of well- developed alimentary canal with only one opening called mouth (Fig. 14.6A). Hence the alimentary canal is incomplete as the anus is absent.
The alimentary canal is divided into 3 parts:
(i) Mouth
(ii) Pharynx and
(iii) Intestine.
(i) Mouth:
The mouth is situated in the mid- ventral line of the body (Fig. 14.2) and leads through a small buccal cavity into a large cylindrical chamber called pharyngeal pouch or pharyngeal cavity. It is muscular and thick walled (Fig. 14.6A).
(ii) Pharynx:
The pharynx can be everted through the mouth as a proboscis and can be extended to some length. In retracted condition the pharynx remains enclosed in a muscular sheath. The pharynx is tubular in shape with a narrow lumen. The pharynx opens into the intestine.
(iii) Intestine:
The intestine is divided into three main branches. One of these branches runs forward along the middle line and the other two run backwards. Each of those main branches gives off lateral ramifying branches and as a result, the whole intestine forms a network occupying the major part within the body. All the branches end blindly and there is no anal aperture. Columnar epithelial cells line the inner walls of the intestine.
The histological structure of the pharynx is interesting. It is lined with the nucleated parts of the ciliated epithelial cells of the body wall. These nucleated parts invaginate through the basal membrane and muscle layers still having contacts with their own cell bodies by protoplasmic bridges.
Mechanism of Feeding:
Food:
Planarians feed on dead or living organisms, mostly crustaceans, nematodes, rotifers, insects and also fragmentary animals or plants.
Feeding:
By chemo-receptors they can detect their food from certain distance. After detection the planarians capture their prey by entangling with the secretion of the expelled rhabdites (Fig. 14.5B). They engulf the food by enclosing it in the everted pharynx or by suction.
Digestion:
Digestion is initially extracellular, using enzymes secreted by pharynx or gland cells of the intestine. The partially digested food material is then engulfed by phagocytic cells and digestion completed intracellularly.
Egestion:
As there is no anus, undigested food particles are obviously forced out through mouth. The excess food is stored as fat and protein in the intestinal cells and parenchyma.
6. Respiratory System of Dugesia:
Planarians are active animals and need supply of oxygen and removal of carbon dioxide. There is no special system for this purpose. This gaseous exchange is carried out by the whole body surface, i.e. respiratory exchange is by diffusion.
7. Excretory System of Dugesia:
Excretory system in Dugesia is proto- nephridia which consists of a network of tubules that run through the mesenchyme of the whole body length, with a large number of specialized excretory cells, called flame cells.
Excretory tubules:
There are two pairs of excretory longitudinal canals or trunks, one pair on each side which run through the mesenchyme of the whole body length. Each canal is much coiled and is connected with each other by transverse vessels in the anterior region but the longitudinal canals remain separate at the posterior region.
Each main trunk gives off numerous branches within the body. From these branches extremely fine capillary vessels emerge. Many of these capillary vessels bear multi-ciliated terminal cells called flame cells. The longitudinal excretory canals lead to the exterior by minute pores, called nephridiopores.
Flame cell:
Each flame cell is a more or less club-shaped cell bearing many cytoplasmic projections which are diffused in the surrounding mesenchyme. A nucleus is centrally placed in the cytoplasm at the base of a bundle of cilia or flagella. The cytoplasm bears secretory droplets and minute vacuoles.
Each flame cell is hollowed out to form a central cavity or lumen which is continuous with the finer tubule. A bunch of vibratile cilia hangs down into the central cavity and each cilia develops from each basal granule.
Between the wall of central cavity and cilia or flagella there is a ring of microvilli. It is assumed that the microvilli act as valves which control the influx of fluid. It is the flickering movement of these cilia that gives the flame cell—its name.
Physiology:
The excretory products in fluid state enter from the parenchyma (mesenchyme) by diffusion. The cilia or flagella within the cavity of cytoplasm beat continuously which creates a negative pressure in the tubule that draws fluid through the thin wall region of the flame cells and is ultra- filtered.
The large molecules in the fluid such as proteins are excluded because these molecules are unable to pass through openings of the wall and the rest of the excreted fluid is passed into the longitudinal excretory canals and goes out through the nephridiopores.
8. Nervous System of Dugesia:
Planarians possess highly organised nervous system than the diffused nerve-net of cnidarians (Fig. 14.6B). The nervous system consists of a pair of cerebral ganglia joined to form a bilobed brain.
The brain is made up of connecting transverse fibres and nerve cells. It gives short nerves to the eyes and two longitudinal nerve cords which pass backward. From these nerve cords numerous transverse branches are given both to the external and internal parts of the body.
The internal branches often anastomose to form commissures. Two types of sense organs, eyes and auricular organs, are recognised in the planarians.
(1) Eyes:
They occur as a pair of rounded dark spots on the anterior dorsal surface (Fig. 14.7). The eyes are made up of a cup- shaped pigment screen. Inside the cup there are many sensory cells. The planarians can make crude discrimination of the direction of light.
(2) Auricular organs:
A few sensory cells are found to be arranged in groups at the sides of the head. These cells lack cilia and rhabdites and are called auricular organs. These organs are responsible for chemical sense.
9. Reproductive System of Dugesia:
Dugesia reproduces both asexually and sexually (Fig. 14.8).
Asexual reproduction:
Some planarians reproduce asexually by transverse fission. The worm constricts into two—a little behind the pharynx and each piece grows and regenerates the missing parts. Planarians possess proverbial power of regeneration. In fact any part of the body can grow and develop into a new individual (Fig. 14.8).
Sexual reproduction:
Dugesia is hermaphrodite, i.e. same individual bears male and female reproductive organs (Fig. 14.8B).
(a) Male:
Male reproductive organ consists of testes, numerous vasa efferentia, a pair of vasa deferentia and a penis. The testes are numerous, small and round. They are situated on the right and left borders on the body. Each testis is connected to the vas deferens of its side through a small duct, the vas efferens.
The right and left vas deferens unite at the middle of the body and form a median duct which passes through the muscular penis. The penis opens into the genital atrium which opens to the outside on the mid-ventral line beneath the mouth. The median duct is somewhat swollen at the base of the penis and forms the vesicula seminalis. The median canal receives unicellular prostate glands.
(b) Female:
Female reproductive organ consist of a pair of small, rounded ovaries or germaria situated at the anterior end of the body. From each ovary there arises one oviduct.
The two oviducts meet at the posterior and ventral part of the body to form a common oviduct which opens into the genital atrium. The genital atrium is connected with a median round chamber—the uterus and a thick-walled muscular structure—the muscular sac. Numerous branching vitelline glands (yolk glands) open into the oviducts.
Process of sexual reproduction:
Though hermaphrodite yet the planarians practise cross-fertilization. During sexual reproduction two planarians bring their postero-ventral surfaces together and copulate. Copulation here means mutual exchange of sperms. Sperms from the seminal vesicle of the male system of one are passed onto the female genital atrium of the other.
The planarians then separate and the sperms migrate to the oviduct to fertilize the eggs. The fertilization is internal. Several zygotes together with yolk cells later become encased in a capsule or egg shell and then the egg shell comes outside the body of the mother. Development is direct and there is no larval form.
10. Regeneration in Dugesia:
Dugesia and some related planarians possess the remarkable power of regeneration. If an animal is cut transversely into two halves (Fig. 14.9A) the anterior half containing head can easily regenerate the posterior part including a tail and a full animal can be developed.
Again the posterior half can develop a new head anteriorly and a new planarian can be developed by making up the lost parts of the individual. Regeneration can be effected by the formative cells of parenchyma.
The regeneration parts of the animal also possess the linear polarity because the anterior part always contains head and posterior part contains the tail. If the anterior end of Dugesia is cut longitudinally along the mid-line (Fig. 14.9B) each part of the head will develop into a complete head, and several longitudinal pieces of the head will develop into several complete heads, thus shows the lateral polarity.
Regeneration is effected by two processes:
(i) Epimorphosis and
(ii) Morphollaxis.
Epimorphosis is a process in which the lost parts of the planarians are formed and morphollaxis is related to the adjustment and function of the original parts with the regenerated parts.
The power of regeneration is maximum in the head region and gradually decreases towards the tail region. This power of regeneration is related to the metabolic rate of the different parts of the body. The metabolic rate is highest in the anterior region and lowest in the tail or posterior region. This variation of metabolic rate with the regenerative power is called the axial gradient.