CLASS POLYCHAETA (Poly, many + chaise, hair)
Polychaeta is the largest of the annelid classes. Polychaetes are mostly marine. They are 5 to 10 en– long. They have more than 5.300 species. Polychaetes are adapted to a variety of habitzts. Many live on the ocean floor, under rocks and shells. Some live within the crevices of coral reefs. Other polychaetes are burrowers. They move through their substrate by peristaltic contractions of the body wall. Other polychaetes construct tubes of sand trains or secreted organic materials. Mucus lines the tubes. This mucous is used tbr protective retreats and feeding.
EXTERNAL STRUCTURE AND LOCOMOTION
Parapodia and setae
The polvchaetes develop parapodia. Parapodia are lateral extensions of the body wall. Chitinous rods support the parapodia. Numerous setae project from the parapodia. The name of class is derived form presence of numerous setae. Setae are bristles structures. They are secreted by the invaginations of the distal ends parapodia. The setae dig into the substrate. Therefore they help in locomotion. Setae also hold a worm in its burrow or tube.
Prostomium (pro, before + stoma, mouth)
The prostomium of a polychaete is a lobe like structure. It projects dorsally and anteriorly from the mouth. It contains numerous sensory structures like eyes, antennae, palps. It also contains ciliated pits or grooves called nuchal organs. The first body segment is peristomium. It surrounds the mouth and it bears sensory tentacles or cirri.
The epidermis of polychaetes consists of a single layer of columnar cells. These cells secrete a protective, nonliving cuticle. Some polychaetes have epidermal glands that secrete luminescent compounds.
Fig: Polychete Locomotion. (a) Dorsal view of a primitive polychaete, showing the antagonism of longitudinal muscles an opposite sides of the (b) Both the period and amplitude of locomotor waves increase as ‘a potychaete changes from a slow walk to a swimming mode.
Mechanism of locomotion
Various species of polychaetes can walk, fast crawl, or swim. Their longitudinal muscles of one side of the body act antagonistically to the longitudinal muscles of the other side of the body. Thus they produce undulatory waves. These waves move along the length of the body from the posterior end toward the head. The parapodia and setae act against the substrate or water and they produce propulsive force. The longitudinal muscles of one side of the segment contracts. The muscles of parapodia of that side lso contract. It stiffens the parapodia. Therefore it protrudes the setae for the power stroke. The period and amplitude of undulatory waves increases when polychaetes changes from a slow crawl to swimming.
Burrowing polychaetes push through sand and mud by contractions of the body wall. They make burrow during eating substrate. For this purpose, the polychaetes digest organic matter in the substrate. It eliminates absorbed and indigestible materials through the anus.
FEEDING AND THE DIGESTIVE SYSTEM
The digestive tract of polychaetes is a straight tube. Mesenteries and septa suspend the digestive tube in the body. The anterior region of the digestive tract is modified into a proboscis . The proboscis has special protractor muscles. These muscles and coelomic pressure -an protrude the proboscis through the mouth. Retractor muscles bring the proboscis back into the peristomium. In some polychaetes the proboscis protrudes and paired jaws are opened. These jaws are used for capturing prey. Predatory polychaetes do not leave their burrow or coral crevice. When prey approaches a burrow entrance, the worm quickly extends its anterior portion. It pulls the prey back into the burrow. Some polychaetes have poison glands at the base of the jaw. Other polychaetes are herbivores and scavengers and use jaws for tearing food. Deposit-feeding polychaetes ingest organic matter from the marine sediments.
The digestive tract consists of a pharynx a crop (storage sac) and a grinding gizzard. They have a long, straight intestine. These are similar to the digestive organs of earthworms. Organic matter is digested out side the cells. The inorganic particles are passed through the intestine and released as castings.
Many sedentary and tube-dwelling polychaetes are filter feeders. They lack a proboscis. But they possess other specialized feeding structures. Some tube dwellers like fan worms, possess radioles. Radioles form a funnel-shaped fan. Cilia on the radioles circulate water through the funnel shaped fan and trap the food particles. Trapped particles are carried through groove at the axis of the radiole. A sorting mechanism rejects the largest particles and transports the small particles to the mouth. Another filter feeder, chaetopterus lives in a U-shaped tube. It s cretes a mucous bag. Mucous bag collects food particles which are I pm small. The par odia of segments 14 through 16 are modified into fans. This fan creates filtration urrents. Finally the mucous bag is ingested.
Elimination of digestive waste is a problem for tube-dwelling polychaetes. Some polychaetes live in-tubes that open at both. Circulating water carry away the waste. Some live in tubes that open at one end. They must turn around tube to defecate . They use ciliary tracts to carry feces to the tube opening.
Some polychaetes live in substrates rich in dissolved organic matter. They can absorb 20 to 40% of their energy requirements through their body wall in the form of sugars and other organic compounds.
GAS EXCHANGE AND CIRCULATION
Respiratory gases diffuse through the body wall. The parapodia increase the surface area for gases exchange. In many polychaetes, parapodial gills further increase the surface area for gas 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:
- Dorsal aorta: The dorsal aorta of polychaetes circulatory systems acts as contracting elements. Dorsal aorta lies just above the digestive tract. They propel blood from rear to front. In the front, the blood moves into ventral aorta.
- Ventral aorta: It is present ventral to the digestive tract. It propels blood from front to rear.
- Segmental vessels: Two or three sets of segmental vessels are present between dorsal and ventral vessels. Segmental vessels receive blood front the ventral aorta and break into capillary beds in the gut and body wall.
- Capillaries: Capillaries unite– again into segmental vessels. These vessels deliver blood to the dorsal aorta.
NERVOUS AND SENSORY FUNCTIONS
The annelid nervous system includes a pair of suprapharyngeal ganglia. It is connected to a pa r of subpharyngeal ganglia by circumpharyngeal connectives. These connectives run dorsoventrally along both side of the pharynx. A double ventral nerve cord run throughout the length of the worm. Ventral nerve cord has a paired segmental ganglion is in each segment. Lateral nerves emerge from each segmental ganglion. These nerves connect muscles of the body wall and other structures of that segment. Followings are functions of different parts of nervous system.
- Segmental ganglia: They coordinate in swimming and crawling movements. Each segment acts separately. But they are closely coordinated with the neighboring segments.
- Subpharyngeal ganglia: They coordinate the distant segments for locomotion.
- Sup pharyngeal ganglia: They control motor and sensory functions. These functions are feeding and sensory functions associated with forward locomotion.
- Ventral nerve cord: It has small-diameter fibers. These fibers coordinate locomotion. The ventral nerve cord also contains giant fibers. These giant fibers are involved in escape reactions. For example. a harsh stimulus like fish hook causes rapid withdrawal from the stimulus. Giant fibers are approximately 50 m in diameter.
They conduct nerve impulses at 30 in/second.
Fig: (a)Nervous System of a Polychaete. (a) Connectives link suprapharyngeal ganglia. (b) Cross section of the ventral nerve cord, showing giant fibers.
1. Eyes: Polychaetes have various sensory structures. Two to four pairs of eyes are present on the surface of the prostomium. They vary in complexity. They may be a simple cup of receptor cells. Or they may have structures made up of a cornea, lens. and vitreous body. Most polychaetes react negatively to increasing light intensities. Fan worms react negatively to decreasing light intensities. If shadows cross them, fan worms retreat into their tubes. This response protects fan worms from passing predators.
2. Nuchal organs: These are pairs of ciliated sensory pits or slits in the head region. Nerves from the suprapharyngeal ganglia supply nuchal organs. Nuchal organs are chemoreccptors for food detection.
3. Statocysts: They are present in the head region of polychaetes.
4. Tactile senses: Ciliated tubercles. ridges, and bands contain receptor for tactile senses. They cover the body wall.
Fig: Annelid Nephiridia. (a) Protonephridium. (b) Metanephridium (c) In modern annelids, the gonoduct and the nepbridial tubules undergo varying degrees of fusion. (d) Nephridia of modern Annelids.
Annelids excrete ammonia. Ammonia diffuses readily into the water. Nitrogen excretion occurs through the body wall. Excretory organs of annelids are more active in regulating water and ion balances. Most marine polychaetes cannot survive in dilute marine environments. It cause osmotic influx of water and the resulting loss of ions. Efficient osmoregulatory abilities are present in a few polychaetes. Such polycha es are adapted in freshwater.
Excretory organs of annelids are called nephridia. Annelids have two types of nephridia.
1. Protonephridium: It consists of a tubule. One bulb like end of the tube is closed. The other end is connected to the outside of the body. Protonephridia have a tuft of flagella in their bulb end. It drives fluids through the tubule. Some primitive polychaetes possess paired, segmentally arranged protonephridia. Their bulbular end projects through the anterior septum and passes into an adjacent segment. It opens through the body wall by a nephridiopore.
2. Metanephridium: Metanephridium is present in most polychaetes. A metanephridium consists of an open ciliated funnel called nephrostome. Nephrostome projects through an anterior septum into the coelom of an adjacent segment. The other end of tubule opens through the body wall at a nephridiopore. Sometimes, it may open through the intestine. There is one pair of metanephridia per segment. It tubules is extensively coiled. Its posterior end dilated into a bladder. A capillary bed is present around the tubule of a metanephridium. It is used for active transport of ions between the blood and the nephridium.
Some polychaetes also have chloragogen tissue. They are associated with the digestive tract. This tissue functions in amino acid metabolism in all annelids.
REGENERATION, REPRODUCTION, AND DEVELOPMENT
1. Asexual reproduction
All polychaetes have remarkable powers of regeneration. They can replace lost parts of their body, Some species have breaking points. These breaking points allow the worms to break themselves when a predator captures them. Lost segments are later regenerated. Some polychaetes reproduce asexually by budding or by transverse fission.
2. Sexual reproduction
Sexual r production is more common in them. Most polychaetes are dioecious.
Gonads develop masses of gametes. They project from the coelomic peritoneum. Gonads occur in every body segment in primitive animals. But most polychaetes have gonads limited is specific segments. Gametes are shed into the coelom. They get mature there. Mature female worms are packed with eggs. Gametes enter the nephrostome of metanephridium. These are released through the nephridiopore or they may be released after the rupturing of worm. In these cases, the adult soon dies. Only a few polychaetes have separate gonoducts. It is primitive condition.
Fertilization is external in most polychaetes. A few species copulate and fertilization is internal. A unique copulatory habit is found in Platynereis megalops. The male and female worms stop feeding at the end of their lives. Their intestinal tracts degenerate. Gametes accumulate in the body cavity. The male and female worms coil together for transferring sperm. The male inserts its anus into the mouth of the female. The digestive tracts of the worms have degenerated. Therefore, sperm transfer directly from the male’s coclom to the egg-Idled coelom of the female. Fertilization of a large number of eggs takes place by this method, Then the female sheds eggs from her anus. Both worms die soon after copulation. The formation of a reproductive individual is called Epitoky. An epitoke is difterent from the non reproductive form of the species called an atoke. The body of epitoke is modified into two body regions. Anterior segments carry on normal maintenance functions. But the posterior segments are enlarged and tilled with gametes. The epitoke may have modified parapodia for more efficient swimming.
Release of many gametes by a large number of male and female at the same time is called swarming. Swarming takes place in many of the polychaetes like Eunic. Zoologists believe that swarming of epitokes does three things.
1. The non reproductive individuals remain safe below the surface waters. Therefore predators cannot destroy an entire population.
2. The individuals become reproductively active at the same time tor external fertiilzation. They must also be closer to one another. Swarming ensures that large members of individuals are present in the right place at the right time.
3. Swarming of vast numbers of individuals for brief periods provides a great chance to the predators. However, vast numbers of prey are available for only short periods during the year. Therefore, predator populations cannot increase beyond the limits of their normal diets. The predators still leave epitokes that will yield the next generation of animals.
Spiral cleavage takes place in fertilized eggs. It produces trochophore larvae. Trochophore larva budded to form segments from anterior to the anus. Larvae settle on the substrate. New segments are continuously added posteriorly. Thus, the anterior end of a polychaetes is the oldest end. Many other polychaetes lack a trochophore. Therefore, they hay direct development. Some are metamorphosed from another larval stage.