CLASS OLIGOCHAETA (oligos, few + chaete, hair)
The class Oligochaeta has over three thousand species. They are found throughout the world in freshwater and terrestrial habitats. A few oligochaetes are estuarine, some are marine. Aquatic species live in shallow water, where they burrow in mud and debris. Terrestrial species live in soils some live in hot, dry weather. The depths of their burrow are 3 m below the surface.
EXTERNAL STRUCTURE AND LOCOMOTION
Oligochaetes have lesser number setae of than polvehaetes. The parapodia and long setae interfere with their bumming lifestyles. Therefore, oligochaetes lack parapodia. But they have short setae on their body wall. The prostomium consists of a small lobe or cone in front of the mouth. Prostomium lacks sensory appendages. A series of special segments are present at the anterior half of Oligochaeta. These segments swell to form a girdle like structure called the clitellum. Clitellum secretes mucus during copulation. It also forms a cocoon. They secrete a nonliving cuticle over the body.
Locomotion takes place by antagonistic contraction of circular and longitudinal muscles. The waves of contraction move from rear to front. The longitudinal muscles contract and segments bulge and setae protrude out. Therefore, the setae penetrate into the soil. The contraction of circular muscles retracts the setae. It elongates the segments and pushes them forward. Contraction of longitudinal muscles in segments behind a bulging region pulls those segments forward. The waves of muscle contraction move anteriorly on the worm. Therefore the segments move forward.
The coelomic hydrostatic pressure helps in burrowing. This pressure is transmitted toward the prostomium. Earthworm uses its expanded posterior segments. It pushes through the soil during burrowing. It uses its protracted setae to anchor itself to its burrow wall. Contraction of circular muscles makes the prostomium a conical wedge. It is I mm in diameter at its tip. Contraction of body-wall muscles generates coelomic pressure. This pressure forces the prostomium through the soil.The earthworms swallow a considerable quantity of soil during burrowing.
FEEDING AND THE DIGESTIVE SYSTEM
Oligochaetes are scavengers. They feed on fallen and decaying vegetation.
They drag these decay food into their burrows at night. The digestive tract of oligochaetes is tubular and straight.
1. Mouth: The mouth opens into a muscular pharynx.
2. Pharynx: The pharyngeal muscles attach to the body wall. The pharynx acts as a pun p tbr ingesting food. The pharynx pumps the food into the oesophagus.
3. Oesophagus: The esophagus is narrow and tubular. It expands to form a stomach, crop or gizzard. Crop and gizzard are common in terrestrial species.
4. Crop: A crop is a thin-walled storage structure.
5. Gizzard :A gizzard is a muscular structure. Its inner wall is lined with cuticle. Gizzard is grinding structure. Calciferous glands are present in the invagination of esophageal wall. They are used to remove excess calcium absorbed from food. They also help to regulate the pH of body fluids.
6.Intestine: The intestine is a straight tube. It is the principal site of digestion and absorption. A dorsal fold of the epithelium is presentin the intestine. It is called typhlosole. It increases the surface area of the intestine. The intestine ends at the anus.
GAS EXCHANGE AND CIRCULATION
Respiratory gases simply diffuse across the body wall. The parapodia increase the surface area for gases exchange.
Polychaetes have a closed circulatory system. Oxygen is carried by respiratory pigments. The respiratory pigments are dissolved in the plasma. They are present in the blood cells in other animals. Blood is colorless. green, or red, depending on the respiratory pigment. It has following circulation pattern:
1. Dorsal aorta: The dorsal aorta of polychaetes circulatory systems acts as contracting elements. Dorsal aorta lies just above the digestive tract. It propels blood from rear to front. In the front, the blood moves into ventral aorta.
2.Ventral aorta: It is present ventral to the digestive tract. It propels blood from front to rear.
3. Hearts: Some segmental vessels expand and they are contractile the earthworm. “Fhese are sometimes called hearts. They transfer the blood from dorsal to ventral blood vessels.
4. Segmental vessels: Two or three sets of segmental vessels are present between dorsal and ventral vessels. Segmental vessels receive blood from the ventral aorta and break into capillary beds in the gut and body wall.
5. Capillaries: Capillaries unite again into segmental vessels. These vessels deliver blood to the dorsal aorta.
NERVOUS AND SENSORY FUNCTIONS
(Seethe nervous system in Polychaete)
The ventral nerve cords and all ganglia of oligochaetes show a high degree of fusion. The giant fibers produce escape responses. An escape response results from the stimulation of anterior or posterior end of a worm.
Oligochaetes are burrowing animals. Therefore, they lack well-developed eyes. Animals living in continuous darkness do not have well-developed eyes. Other oligochaetes have simple pigmented cup called ocelli. Photoreceptor cells scattered over the dorsal and lateral surfaces of the body. Therefore, they all have a “dermal light sense”. Photoreceptor cells produce a negative phototaxis in strong light and a positive phototaxis in weak light.
Oligochaetes are sensitive to different chemical and mechanical stimuli. Receptors of these stimuli are scattered over the body surface, especially around the prostomium.
Oligochaetes use metanephridium for excretion and osmoregulation (ion water balance). The nephrostome of metanephridium is present in anterior segment. Their tubule and nephridiopore are present in the posterior segment. Nitrogenous wastes are ammonia and urea. Oligochaetes excrete large amounts of very dilute urine. But they retain vital ions. It is important for them. They live in environments where water is plentiful but essential ions are hinted.
Oligochaetes possess chloragogen tissue. These tissues surround the dorsal blood vessel. They lie over the dorsal surface of the intestine. Chloragogen tissue acts as vertebrate liver. It is a site of amino acid metabolism. Chloragogen’ tissue deaminates amino acids and converts ammonia to urea. It also converts excess carbohydrates into glycogen and Fat.
REPRODUCTION AND DEVELOPMENT
Oligochaetes are monoecious. They exchange sperm during copulation. One or o pairs of tested and one pair of ovaries are present on the anterior septum of some anterior segment . Both the sperm ducts and the oviducts have ciliated funnels at their proximal end. It i. used to draw gametes into their male or female tubes.
Testes are associated with three pairs of seminal vesicles. Seminal vesicles are used for maturation and storage of sperm. Seminal receptacles in female receive sperm during copulation. A pair of very small ovisac is present. It is associated with oviducts. Egg is stored and become mature in ovisacs.
Fig: Earthworm Reproduction. Mating earthworms, showing arrangements of reproductive structure and the path sperm take during sperm exchange.
Two worms line up facing opposite directions during copulation. Their ventral surfaces and their anterior ends are in contact with each other. This arrangement lines up the clitellum of one worm with the genital segments of the other worm. Clitellum secretes a mucous sheath. This sheath covers the anterior halves of both worms. This sheath holds the worms in place. Some species also have penile structures and genital setae. These structures maintain contact between worms. The Sperm duct releases sperm. Sperms travel in a groove in the external ventral body wall. This groove is formed by the contraction of special muscles. Muscular contractions along this groove propel the sperm toward the openings of the seminal receptacles. In other Oligochaetes, there is direct transfer of sperm into seminal receptacle during copulation. Copulation lasts for two to three hours and both worms exchange sperms.
The clitellum forms cocoon for the deposition of eggs and sperm. The cocoon consists of mucoid and chitinous materials. The clitellum secretes albumen into the cocoon. The worm begins to come out of the cocoon. The cocoon passes through the openings of the oviducts and eggs are deposited in it. The sperm are released as the cocoon passes the openings to the seminal receptacles. Fertilization occurs in the cocoon. The worm come out from the cocoon. The ends of the cocoon are sealed. The cocoons are deposited in moist soil.
Spiral cleavage is modified. Larva is not formed in them. Hatching occurs in one to few weeks. The young worms emerge from one end of the cocoon.
Freshwater Oligochaetes also reproduce asexually. Asexual reproduction takes place by the transverse division of a worm. The missing segments are regenerated.