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Phylum Platyhelminthes: Acoelomated Animals General Characteristics Bilateral symmetry: body can be divided along one plane of symmetry to yield 2 mirrored halves (anterior, posterior, dorsal, ventral, left and right) Has 1 digestive cavity Acoelomates: lack a coelom (body cavity) Exhibit cephalization: has a head region with sensory structure Triploblastic: contain 3 well defined germ layers Has mesoderm: region between endo- and ectoderm made of parenchyma Mesoderm enables the formation of some internal organs (muscular, excretory, and reproductive) Mesoderm also gives rise to the Parenchyma- a form of solid tissue containing cells and fibers Reach the organ system level of organization Has a central nervous system Platyhelminthes “Firsts” Bilateral symmetry Has digestive cavity Triploblastic Reach the organ system level of organization Cephalization Central nervous system First with an excretory system How are Platyhelminthes different from cnidarians? Cnidarians Platyhelminthes Mesoglea Mesoderm No organ systems Diploblastic GVC Radial Symmetry Not cephalized Organ systems Triploblastic Digestive cavity Bilateral Symmetry Cephalized Nerve net Nervous system with brain Cnidocytes No cnidocytes Gonads Complex reproductive system General Structure: Epidermis Most general characteristics are describing Turbellaria class Epidermis: made from the ectoderm ◦ Ciliated: especially on the ventral surface to aid in movement ◦ Contain many gland cells that secrete a mucous covering to help move Epidermis has rhabdites: rod-like structures in the cells of the epidermis that are discharged and form a protective mucous covering by expanding when released General Structure: inside the epidermis Contains layers of muscle cells (fibers) ◦ Run circularly, longitudinally, and diagonally ◦ Come from the mesoderm Parenchyma cells: ◦ Develop from the mesoderm ◦ Fills the space between the muscle cells and the visceral organs ◦ Layer that prevents them from being coelomates Locomotion Movement is a concerted effort of ciliary action, mucous, and muscular movements Most movement accomplished by Turbellaria …. Why? The head (superior) is raised while the other portion (inferior) is kept against the surface Glides slowly Anterior end bends from side to side Nutrition and Digestion Class Cestoda are the exception to the following notes on nutrition and digestion Cestodes have no digestive tract because they are parasites that depend on their host for nutrients- they absorb nutrient from their host Other classes have: Mouth: near the middle of the ventral surface; opens to the pharynx Pharynx: tubular muscular organ Contains many gland cells Can be extended from the mouth during the feeding process Secrete enzymes to help soften prey Sucking action from the pharynx breaks off small pieces of the prey Intestine Tri-clad Intestine (3 trunks)= class Turbellaria Bi-clad (2 Trunks)= Class Trematoda No digestive tract= Cestoda Excretion and Osmoregulation Platyhelminthes are the first organism with an excretory system Consists of Protonephridium: first kidney ◦ System lies in the parenchyma and consists of a network of tubules that run the length of the worm on each side and opens to the surface by minute pores ◦ Side branches terminate in tiny enlargements called flame bulb: cup shaped terminal end that contains flagella to propel the fluid throughout the network to the exit pores (nephridiopores) Primary function of the flame bulb system is to remove the excess water from the tissues Circulatory and Respiratory System Platyhelminthes contain no circulatory or respiratory system Respiration occurs through epidermal cells of the whole body surface (diffusion) To allow enough oxygen to pass in and carbon dioxide to pass out through diffusion-> the thickness can’t be too great This is the reason why the organism is so FLAT!!!! Nervous System Consists of the simplest nervous system called the sub-epidermal nerve plexus Resembles the nerve net of the cnidarians Functions to send the nerve impulses from one end to the other ◦ Consists of a brain, nerve cords, and side branches Brain: a mass of ganglion cells at the anterior end ◦ Centered anteriorly ◦ Not needed for muscular coordination involved in locomotion ◦ Functions to initiate behavior and receives stimuli from sensory organs and sends it to the rest of the body Nerve cord: run from the brain to the posterior Side branches: come off the nerve cord and directed toward the inside of the body Forms a ladder type of system Sense Organs Ocelli: light sensitive eye spots Statocysts Have tactile and chemoreceptive cells over entire body Have organs called auricles (ear-like lobes on the side of their head) ◦ Functions in finding food (chemoreceptive) ◦ If they are cut off they have a hard time finding food Reproduction and Regeneration Most information about class Turbellaria Can reproduce asexually and sexually Asexual reproduction accomplished by fission ◦ The area behind the pharynx constricts and the organism separate into 2 animals ◦ Each part will regenerate missing parts Many species have remarkable powers of regeneration and repair wounds Sexual reproduction ◦ Almost all flatworms are hermaphroditic: contain both male and female sex organs in every organism ◦ Have discrete testes and ovaries ◦ Have a system of tubes and chambers in which fertilization occur ◦ Do not practice self-fertilization-> they reproduce by crossfertilization ◦ After breeding season the organs degenerate and regenerates new at the beginning of the next season Protandry Mutualistic insemination Sperm is produced first and the organisms exchange sperm with another The organism holds the sperm until the eggs are ready Eggs develop later Platyhelminthes Classification Class Turbellaria ◦ Free living planarians & marine flatworms Class Trematoda ◦ Internal Flukes Class Cestoda ◦ Tapeworms Class Turbellaria Most free living flatworms Marine or freshwater Class of the planaria Can regenerate Not parasitic Planaria class are carnivorous- feeding on small crustaceans, nematodes, rotifers, and insects Most have chemoreceptors to trap prey in mucous (rhabdites) The notes previously taken were on class Turbellaria Form Fits Function The idea that the structure of an organism/part of an organism is related to its function Examples: ◦ Location of the mouth on the Platyhelminthes ◦ Lack of a digestive tract in the Cestoda Class ◦ Shape/depth of the planaria ◦ Turbellaria is the only class highly motile