Polychaeta Quiz

Polychaeta

• Bristle worms live in marine environments, frequently in shallow intertidal zones.
• Many are carnivores, feeding on invertebrates.
• Body has numerous chaetae.
• A head region with eyes, tentacles, and palps is present.
• Each segment bears one pair of parapodia.
• A clitellum is absent.
• Sexes are separate, and development is indirect, involving a trochophore larva.

Sand Worm (Nereis)

• Body divided into many segments (200+).
• Segments separated by intersegmental grooves.
• Well-developed head region, consisting of:
  • Prostomium: Carries the mouth and sensory organs (4 eyes, 2 tentacles).
  • Rest of the body bears parapodia.

Parapodia

• Each parapodium consists of a dorsal notopodium and a ventral neuropodium.
• Dorsal notopodium has a dorsal cirrus with a chaetae bunch.
• Ventral neuropodium has a ventral cirrus with a chaetae bunch.
• Parapodia are supported by dorsal and ventral acicula (long setae).

Parapodium Functions

• Locomotion.
• Respiration (increase respiratory surface).
• Sensation.

Nereis Internal Anatomy

• Body wall with muscles (circular and longitudinal).
• Coelomic epithelium..
• Dorsal and ventral blood vessels.
• Digestive system (mouth, pharynx, esophagus, crop, stomach, intestine, anus).
• Nephridia (excretory organs).
• Nerve cord.
• Eggs.
• Respiratory capillaries.
• Setae.

Nereis Feeding and Digestive System

• Carnivorous, feeding on crustaceans.
• Mouth is below the prostomium, leading to a muscular pharynx.
• Pharynx is lined with thickened cuticle.
• Numerous small teeth and two jaws are thickened in specific areas of the pharynx.
• Prey is captured by mouth jaws.
• Teeth pull food into the pharynx.

Nereis Digestive System Details

• Pharynx leads to a narrow esophagus with bilobed esophageal glands.
• Followed by a long intestine without a typhlosole.
• Ends with rectum and anus.

Nereis Excretory System

• One pair of nephridia per segment (except some anterior and posterior ones).
• Each nephridium consists of a syncytial mass with a coiled nephridial tubule opening at both ends.
• Nephrostome (opening in coelom) is in anterior segment.
• Nephridiopore (opening to the outside) is ventral at base of parapodium.
• Nitrogenous waste (mainly ammonia) is collected from coelomic fluid and blood.
• Cilia move waste to the outside via nephridiopores.

Nereis Blood Vascular System

• Dorsal blood vessel.
• Ventral blood vessel.
• Parapodial capillaries.
• Perineural blood vessels.
• Dorso-intestinal vessel.
• Ventro-intestinal vessel.
• Intestinal capillaries.

Reproduction and Life Cycle

• Sexes are separate.
• Gametes bud from gonads and mature in coelom.
• Gonoducts and gonopores are absent.
• Males mature near the surface, attracting females.
• Segments with gonads rupture to release eggs and sperm into water.
• Fertilization occurs in water, forming zygotes.
• Zygotes develop into trochophore larvae.

Trochophore Larva

• Has preoral ciliated ring used for swimming.
• Apical sensory plate with cilia.
• Complete gut with mouth and anus.
• Nephridia and mesodermal cells are present in the larva.
• Cilia are lost, and the larva settles to the bottom to become a young worm.

Hirudinea (Leeches)

• Live in aquatic and damp terrestrial environments.
• Fixed number of segments, subdivided into secondary annuli.
• Reduced head, no tentacles.
• No chaetae or parapodia.
• Terminal suckers (for attachment and locomotion).
• Sexes are separate, and development is direct in a cocoon.

Hirudo Medicinalis (Medical Leech)

• Freshwater annelid.
• Blood-feeding ectoparasite.
• Two suckers.
• Anterior sucker encloses the mouth and has 3 jaws.
• Posterior sucker is non-perforated.
• First five segments have 5 pairs of eyes.
• Remaining segments have papillae.
• Body consists of 33 segments but divided into 5 annuli (variaion between anterior and posterior).
• Nephridiopores (17 pairs) open ventrally; anus in front of posterior sucker.
• Male genital opening on segment 11; female on 12.

Leech Locomotion

• Uses body movements and suckers to move.

Leech Digestive System and Nutrition

• Modified for blood-feeding.
• Mouth with 3 jaws to cut skin.
• Suctorial pharynx.
• Crop with lateral diverticula to store blood.
• Stomach.
• Intestine, opening externally via anus.
• Salivary glands secrete hirudin (anticoagulant).

Leech Respiration and Excretion

• Respiration occurs through skin.
• Excretion is performed by 17 pairs of nephridia in segments 7-23.

Leech Reproductive System

• Hermaphroditic (both male and female reproductive organs present).
• 9 pairs of testes.
• Common vas deferens, epididymis.
• Male genital opening on segment 11.
• Two ovaries, oviducts, vagina.
• Female genital opening on segment 12.
• Cross-fertilization common.
• Fertilized eggs are laid in cocoon for development.
• Direct development.