Lecture biology subject Nemertea

Nemertea • 1150 species of ribbon worms, most of them in marine, only 12 species in freshwater, 15 terrestrial species • Free living, predators with proboscis Form • Long slender worms, several species exceeding 1m, even 54m (lineus longissimus) • The anterior end is pointed, rounded or spatula shaped • Depending on taxon, the body has different colour: yellow, orange, red and green… Body wall, locomotion, and extensibility • Body wall composes: - Epidermis with cilia and microvili - Musculature with layers (circular, longitudinal and Diagonal or helical muscles) - A layer of connective tissue lies between the musculature and gut (gelatinous in pelagic species, mesenchyme in other) The body wall of nemertean • Locomotion Nemerteans can move by the combined action of their musculature and cilia: - Gliding over substratum on the trail of slime thanhks to epidermal cilia - Crawling over surfaces with the help of muscular waves to or muscular peristalsis (burrowing nemerteans) - Swimming by the action of dorsoventral undulation of body - Protruding the proboscis, attaching themselves by means of the stylet, and then drawing their body forward (some species) • Extensibility Most nemerteans are extensible worm and can extend to at least 10 times the resting body length Proboscis and Rhynchocoel • The shape of proboscis: - Long, extensible, muscular tube - Lying in a fluid-filled coelomic cavity, called rhynchocoel Proboscis and Rhynchocoel • Function: - used to capture preys and move The proboscis evertes (turn inside out) as muscles of rhynchocoel wall compresse the rhynchocoel cavity Proboscis eversion Stylet apparatus in the proboscis • Some nemerteans bear calcarious stylet (barb), attached to proboscis wall by a bulbou structures (basis)to attack and capture preys • Nutrition and digestive system - Nemerteans are carnivores or scavengers - using their proboscis to capture preys, the toxis secretions of proboscis cause preys paralytic before being ingested Nemertean captures preys (sea snails) - Their preys: worms, clams, crustaceans - The ways of capturing preys very complex • Digestive system consists of: - Mouth, forgut, stomach, intestine, and anus - Foregut: buccal, pharynx - Intestine: many lateral diverticula or caeca Digestive system of nemertean - In some nemerteans the mouth disappeared, the pharynx unites with the rhynchodeum - First, extracellular digestion in gut lumen then intracellular by gastrodermal cells Gas exchange, internal transport, and excretion • gas exchange: Across the surface of body • internal transport: coelomic circulatory system cosists of two components: - Central rhynchocoel – its fluid transports substances to and from the proboscis - Peripheral vessels (blood vascular system) transport substances throughout the body Circulatory and excretion system of Nemertean • The feature of circulatory: very simple structure - Two or three longitudinal vessels connected transverse vessels in some parts - Heart is absent - Circulation is intermittent depending on contraction of vessels and masculature - Corpuscles is usually colorless, in some species, the cells are yellow, orange, green, or red (hemoglobin) Circulatory system of nemertean • Excretion - Protonephridia, but restricted to the anterior forgut, not scattered throughout the body as in flatworm - Terminal cells project into the wall of lateral vessels Circulatory system of Nemertean Cross section of protonephridia • Nervous system and sense organs Nervous system: type of orthogon, cosists of : - A brain, a ring of four ganglia - Two lateral large nerve cords - Additional cords (in some cases) Nervous system of nemertean • Sene organs - Sensory epidermal pits - Pigment cup ocelli - Ciliated cephalic slits and grooves - Celebral organs - Eversible frontal organs The last three of these organs are probably chemoreceptors Celebral organs Reproduction and Development Reprodution: Asexual (clonal) and sexual reproduction • Asexual: - Fragmentation and regeneration are two stages of clonal reproduction - Ability of body fragmentation to regenerate new worms is great variable • Sexual reproduction - Gonochoric /dioecious animal, simple reproductive systems - Gametes develop from stem cells in the gonads alternating with the intestinal diverticula in each side of the body - Maturating, the gametes escape from the gonads, fertilation in water Reproductive systems Development - Nemerteans have spiral cleavage, determinative development - Most mesoderm arises from mesentoblast (4d) cell - The embryos of almost nemertean taxa develop directly or via the larval stage (called pilidium) - After a free swimming existence, pilidium metamorphoses into juvenile, which differentiates from imaginal disks Pilidium juvenile Summary Bilateral symmetry Three germ layers Epidermis with cilia and gland cells Body spaces with parenchyma, which is partly gelatinous An eversible proboscis, which lies free in a cavity (rhynchocoel) above the alimentary canal Complete digestive system (mouth to anus) Body-wall musculature of outer circular and inner longitudinal layers with diagonal fibers between the two; sometimes another circular layer inside the longitudinal layer Blood-vascular system with two or three longitudinal vessels Acoelomate, although the rhynchocoel technically may be considered a true coelom 10 Nervous system usually a brain connected to paired longitudinal nerve cords (orthogon) 11 Excretory system of two coiled ducts, which are branched with flame cells (protonephridia) 12 Sexes separate with simple gonads; asexual reproduction by fragementation; pilidium larvae in some 13 No respiratory system 14 Sensory ciliated pits or head slits on each side of head, which communicate between the outside and the brain; tactile organs and ocelli (in some • Similarity with flatworm: - Epidermis is ciliated and has many gland cells - Presence of flame cells in the excretory system - Rhabdites found in several nemerteans ... musculature and gut (gelatinous in pelagic species, mesenchyme in other) The body wall of nemertean • Locomotion Nemerteans can move by the combined action of their musculature and cilia: - Gliding... proboscis • Some nemerteans bear calcarious stylet (barb), attached to proboscis wall by a bulbou structures (basis)to attack and capture preys • Nutrition and digestive system - Nemerteans are... Foregut: buccal, pharynx - Intestine: many lateral diverticula or caeca Digestive system of nemertean - In some nemerteans the mouth disappeared, the pharynx unites with the rhynchodeum - First, extracellular