Download Phylum Platyhelminthesnotesfilledin - Spring

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