Third Lecture: Basic Education - PDF

2- Class: Polycheata  II- Class: Polycheata Habitats: Polychaetes ( bristle worms) live in the marine environment and are very common in the shallow, intertidal zone. Feeding: Many are carnivores and prey on a variety of invertebrates. Characters: 1-They have large number of chaetae. 2- T...

2- Class: Polycheata  II- Class: Polycheata Habitats: Polychaetes ( bristle worms) live in the marine environment and are very common in the shallow, intertidal zone. Feeding: Many are carnivores and prey on a variety of invertebrates. Characters: 1-They have large number of chaetae. 2- There is a head with eyes, tentacles and palps. 3- Each segment carries one pair of parapodia. 4-Clitellum is absent. 5- Sexes are separate and the development is indirect with trochophore larva.  The sand worm (Nereis) - The body is divided into a large number of segments (200 or more) separated by intersegmental grooves. - Well developed head which consist of :- 1) Prostomium carries the mouth, sense organs such as 4 eyes, 2 tentacles and 2 palps. 2) peristomium has 4 pairs of tentacles. - The rest of body segments bear a pair of parapodia. - Each parapodium consists of a dorsal bilobed notopodium and a ventral neuropodium. - Dorsal notopodium bear dorsal cirrus with bunch of chaeta - Ventral neuropodium bear ventral cirrus with bunch of chaeta - The parapodium is supported with two long setae called dorsal and ventral aciculums. Functions of parapodia: - Limps for locomotion - Gills for respiration ( increase the respiratory surface) - For sensation eding and digestive system of Nereis -The worm is carnivorous, feeding on small Crustacea. -The mouth is located just below the prostomium. It leads to a muscular pharynx that can be everted through the mouth. Pharynx is lined with cuticle that is thickened in certain places forming numerous small teeth and two jaws. The prey is gripped by the jaws and drawn inwards as the pharynx is retracted. The teeth help in pulling in the food.  Foreg ut Midgu t Hindg ut -The pharynx leads to a narrow oesophagus that carry bilobed oesophageal gland. -This is followed by long intestine which has no typhlosole and ends with rectum and finally with anus.  EXCRETORY SYSTEM One pair nephridia are found in each segment except a few anterior and a few posterior ones. Each nephridium consists of a syncytial mass of connective tissue and a coiled nephridial tubule which opens at both ends. The nephridial tubule opens by or nephrostome in the coelom of anterior segment. Posteriorly the tube opens by a nephridiopore located ventrally at the base of parapodium. The nitrogenous wastes (mainly ammonia) are collected from coelomic fluid and blood by diffusion and passed by the cilia of nephridial tubules to the outside via the nephridiopores. Useful ood vascular system eproduction and life cycle The sexes are separate. The ova and sperms bud off from the gonads and mature in the coelomic cavity. Gonoducts and gonopores are absent. -At maturity, near the surface, the males are attracted to females. -The segments containing gonads are ruptured shedding the eggs and sperms into the water where fertilization takes place. The zygot develops into trochophore larva. - The larva consists of a preoral ciliated ring by which it can swim in water. An apical sense plate carry cilia is also found and a complete gut with mouth and anus. - Nephridium and mesodermal cells are also present. The larva then loses its cilia and settles down on the bottom transforming into a young worm. Trochophore  3- Class: :CharactersHirudinea Leeches live in both marine and fresh water and in -1.damp terrestrial habitats They have fixed number of segments which -2 subdivided.into secondary annuli.reduced head with no tentacles -3.No chaeta or parapodia -4.Terminal suckers for attachement and locomotion -5.Sex are separate and development is direct in cocoon -6  Hirudo Medicinalis (The medical leech) This is a common freshwater annelid feeding on.blood of vertebrates(ectoparasite) It has 2 suckers, the anterior enclosing the mouth - and 3 jaws while the posterior sucker is not.perforated The first 5 segments carry 5 pairs of eyes, and the -.rest segments carry segmental papillae The body composed of 33 segment and divided - externally into 5 annuli(8-23) , but the anterior.and posterior , the number of annuli is varied Nephridiopores (17 pairs) open ventrally on - - segments(7-23) while the anus opens in front of the posterior sucker. The male genital opening lies on segment 11 while the female one opens on.segment 12 Locomotion  :Digestive The alimentarysystem andsome canal shows nutrition modifications.to the blood sucking habits The mouth encloses 3 serrated jaws ( to cut the skin of host) and followed by suctorial pharynx and a crop extending along 11.segments Crop is provided with a pair of lateral diverticula to increase the storage capacity of.the crop The crop is followed by a small stomach then a narrow intestine which open to the exterior.by the anus The mouth has salivary gland which secrete the :following Hirudin that prevents blood clotting -1.during feeding After filling of its crop with blood , the leech digests blood slowly over a period.of several weeks Respiration and :Excretion Respiration occurs by diffusion throughout the skin Excretion is carried out by 17.pairs of nephridia on segments (7-23) :Reproductive system.The worm is hermaphrodite The male genital system composed of 9 pairs of testes connected with a common vas deferens leading to an epididymis that store sperms and ends with male genital opening on segment.11 The female genital system consists of 2 ovaries, 2 oviducts and vagina that open to the exterior.by the female opening on segment 12 Cross fertilization occurs, sperms enter the vagina of another worm and the fertilized eggs discharged into a cocoon, where the development is completed to form young.worms (direct development)