BIOL 2003: Lesson 5 (Platyhelminthes)

Lophotrochozoa

• at larval stage, all have a trochofore

• all have lophophore feeding structure

Platyhelminthes

• eukaryotes → metazoa → protostomia → lophotrochozoa → Platyhelminthes 

• flatworms

• acoelomate, bilateral, triploblasts, vermiform (meaning worm like body)

• most are parasitic 

• no anus 

• flame cells 

  • used for excretion and osmoregulation 

• well-developed reproduction 

• 4 classes:

  • turbellarians 

  • trematoda

  • monogenea 

  • cestoda 

Organization and development

• bilateral and dorso ventrally flattened

• triploblastic with acoelmoate body plan

• organs present

Flame cells

• earliest excretory system

• filters fluids from inside body

• removes metabolic waste

• controls osmotic pressure

  • retains important ions

• flame cells + tube cells = protonephridia

Protonephridia

• very primitive kidney like structure

Turbellarians

• non-parasitic flatworm

• mostly free living (not requiring a host)

• swim/glide with muscles, cilia, slime 

• feeding in pharynx in ventral centre of body 

• ladder-like nervous system

Turbellarians movement

• swim/glide with muscles, cilia, slime 

  • ciliated epidermis which contains rhabdites (mucus-producing structure) 

  • dual-gland adhesive organs allow for an attach and detach system

Turbellarians digestion

• extracellular and intracellular digestion 

  • extracellular in that they are able to release digestive juices to pre-digest outside of the body

• scavengers/predators 

Turbellarians nervous system

• ladder-like nervous system 

• cephalization

• contains several nerve organs 

  • auricles (chemoreceptive/tactile sense)

  • statocysts (orientation)

  • rheoreceptors (currents)

  • ocelli (eye spots) 

Turbellarians reproduction

• asexual: fissio, regeneration

• sexually: monoecious, meaning they can undergo internal fertilization

Parasitic platyhelminthes 

• syncytial, non-ciliated epidermis

• lacking digestive system in some groups

  • as no need for it, they absorb all food which has already been pre-digested from host

• minimal cephalization

Trematoda

• endoparasitic platyhelminthes, meaning they live inside the host 

• adaptions for parasitism 

  • enzymes, hooks, penetration, cyst forms 

Intermediate host

• fertilized eggs are consumed by the intermediate host, where they hatch, and amplify 

• after amplification, a swimming form goes to infect vertebrate (find a definitive host)

Definitive host 

• final host

• juvenile flukes can infect a final host, where they will then mature and reproduce

• fertilized eggs in feces of definitive host, where they can be taken in by an intermediate host

  • cycle repeats

Common parasitic flukes

• liver flukes

  • undercooked/raw fish

• blood flukes

  • schistosomiasis 

  • directly enters human skin an enters blood vessels and intestines 

  • causes anemia and many indirect symptoms and infections 

Monogenea

• ectoparasitic platyhelminthes (meaning they attach outside of the host) 

  • commonly on the skin/gill of fish 

• attach via opisthaptor

Cestoda

• endoparasitic platyhelminthe (meaning inside the host)

  • attach inside the host via scolex 

• strobila of proglottis (reproductive units)

• no digestive systems 

  • instead, microvilli that increase surface area for absorption of pre-digested food inside the host 

Strobila

• not true segmentation

• reproductive factories 

• youngest at head, most mature at posterior 

• fertilization from same or different strobila 

• eggs our of the uterine pore 

Platyhelminthes organization 

• organs

Platyhelminthes symmetry

• bilateral symmetry 

Platyhelminthes body cavity

• acoelomate 

Platyhelminthes development

• triploblasts 

Platyhelminthes segmentation

• no true segmentation