Annelida (Segmented Worms) - Second Lecture (PDF)

Phylum: Annelida )1 )The segmented worms( Phylum Annelida - Annelida (segmented worms) are group of animals in which their body is formed of many similar rings or segments(annuli). - Occur in marine , brackish , freshwate...

Phylum: Annelida )1 )The segmented worms( Phylum Annelida - Annelida (segmented worms) are group of animals in which their body is formed of many similar rings or segments(annuli). - Occur in marine , brackish , freshwater and terrestrial soil. General characters of Annelida Bilateral symmetry ,triploblastic , coelomate.1.protostomes animals They have soft elongated body with metameric.2 segmentation ; The body is formed of successive segments separated by intersegmental grooves and covered with.cuticle.The digestive tract is complete. Digestion is extracellular.3 4. Respiration occurs by diffusion throughout their body surface. There is a closed circulatory system. The blood is red.5.with haemoglobin The main excretory unit of annelids is the nephridium,.6 opening into the coelom by nephrostome and to the.exterior by nephridiopore The nervous system includes a pair of dorsal cerebral.7 ganglia forming the brain, a pair of peripharyngeal connectives and a solid double ventral nerve cord with a.ganglion and nerves in each segment The sexes are separate or hermaphrodite. Development.8 is direct or indirect (may include a larval stage known as a.trochophore larva)  Classification of Annelida According to the number of chaeta, annelids are classified into three classes in addition to small.unsegmented class: Myzostomia  Class: Class: Class: Polychaet Oligochaeta Hirudinea a Habitat Mostly Mostly Freshwate marine terrestrial r, marine or or freshwater Terrestrial leaches. numerous few chaeta Without chaeta chaeta chaeta  Head Well Reduced and Reduced region developed have with with no tentacles suckers tentacles or sense and sense organs organs Circulator Closed Closed Opened y system Sexes & Separat Hermaphrodi separa e te reproduct te Trochoph Direct development ion ore larva Exampl Hirudo es Nereis Allolopophora (Medical (Sand (Earth leech) worm) worm)  The Earth worm Class: Oligochaeta E.g.: Allolobophora caliginosa The Earth worm :Habitat Worms are nocturnal. They live in damp soil in burrows :Feeding.Feed on decaying organic matter :Worm cast The defecated earth collect on the mouth of burrows :Morphology  Body is cylindrical and divided into segments separated by intersegmental grooves.  Head is poorly developed and formed of peristomium (first segment) and prostomium (forming dorsal lip in front of the mouth)  Themouth end is pointed and located on the peristomium :Morphology  Theanus end is dorsoventrally flattened and located on the last segment.  Clitellum is of thickened skin of segments 26-34.  Dorsalpores open on intersegmental grooves from segment 8 to the end. They connect coelom with exterior  Four pairs of chaetae are present in all segments except the 1st and last one(for anchoring). Each segment except the 1st three and the last one is perforated on the ventral side by a pair of nephridiopores.  Female genital openings found on segment no 14. Male genital openings found on segment 15. Three pairs of copulatory papillae are found on segments 9, 10& 11. Two pairs of Spermathecal pores lie in the intersegmental grooves between segment 9\10 and 10\11. - Two puberty crests are found on the lower edges of clitellum(26-34) are at segments 31-33. Locomotion :Digestive system and nutrition The digestive system of earth worm is -:complete and consists of The alimentary canal starts with the mouth that leads to the buccal region at segments.1-2.The muscular pharynx lies at segments 3-5.The oesophagus (a narrow long tube) 6-14 The crop is a rounded structure store food at.segments 15-16 The gizzard is the strongest muscular portion and its wall is thick for grinding the food.found at segments 17-19 The intestine starts from segment 20 till the.end of the body The earth worm feeds mainly at night  on decaying organic matter found in.the soil The food is moistened while passing down the pharynx, oesophagus and.crop The gizzard crushes the food and pass into the intestine where digestion and absorption of digested food.takes place Excretory system and Excretion The excretory system consists of a pair of nephridia in each segment except the 1 st three.and the last one Each nephridium opens in the coelomic cavity by a ciliated funnel (nephrostome) and to the outside by nephridiopore on the ventral.surface of the body Coelomic fluid with wastes enters the funnel by the aid of cilia from the front segment and drives them into the excretory tube, then to the exterior through nephridiopores ;Circulatory system This system is closed, consists of a dorsal blood vessel (contractile) above the gut and none contractile ventral blood vessel below.the gut The two vessels are connected by 6 pairs of.contractile in segments pseudohearts 6-11 The blood flows backward in the ventral vessel and forward in the contractile dorsal. one Pseudohearts pump the blood downwards into.the ventral vessel again Both dorsal and ventral vessels are interconnected with numerous minute blood.capillaries Reproductive system and Reproduction Male reproductive system It composed of two pairs of testes in.segments 10 and 11 The immature sperms separated from the testes are stored in four pairs of seminal vesicles at segments 9, 10, 11.and 12 Two seminal funnels at segments 10 - and 11 lead to common vas deference that opens to the exterior by two male.genital openings at segment 15 B- Female genital system It composed of two ovaries in segments 13 producing eggs which are carried out to the female genital openings on.segment 14 Two pairs of spermathecae store sperms of another worm during.copulation They open to the outside ventral surface by two spermathecal pores at the intersegmental grooves of.segments 9/10 and 10/11 :Fertilization and life cycle Earthworm reproduces sexually by - cross fertilization The two worms lie near each other with - their ventral surfaces and their heads.pointing in opposite directions The clitellum of each worm lying - opposite to spermathecal pore region of.the other worm The sperms are flow out of the male - genital openings along seminal groove to the region of clitellum then forced to spermatheca of the other worm When eggs mature, the clitellum of each worm secretes a cocoon, when the cocoon passes over the female genital opening(seg,14), it receives eggs and when passes over the spermathecal.pores(9-11), it receives sperms Finally, the worm slips out of the cocoon -.and leaves it in the soil Fertilization occurs inside the cocoon to - form the zygote which develops into.young worm The development is direct i.e., the larval -.stage is absent