Platyhelminthes Flashcards
Study Slide Pack Three
Platyhelminthes
- Flatworms
- Acoelomate, dorsoventrally flattened and bilaterally symmetrical
- Most hermaphroditic
- Protostome, spiral cleavage, some determinate growth
- Mesodermal layer develops into
parenchyma, a loose collection of cells - No anus, respiratory or circulatory organs
- Has excretory structures (like rotifers)
- Clear cephalization w/ central nervous system
Platyhelminthes - Free-living Body Plan
- “turbellarians” – 6,500 spp. in a paraphyletic group
- Range from 1mm to 30 cm long
- Flat body enhances SA:V ratio for efficient gas exchange
- Vulnerable to desiccation
- Has hydrostatic skeleton – ciliary gliding with mucus
Mesodermal layer develops into
parenchyma
A loose collection of cells
- Either 1st animal with mesoderm or secondary loss of coelomic cavity leading to acoelomate condition
Cephalization with sensory structures
auricle, pits, tentacles, and centralized, ladder-like nervous system
Ocellus –
inverted pigment-cup for photoreception
Platyhelminthes - Feeding and Digestion
- Most are carnivorous or scavengers
- Use chemoreception to find food
- Has mouth and pharynx
- Complex intestinal system but incomplete
Simple, triclad (3 branches), polyclad (many)
Gut lined with gland cells and phagocytic cells - Waste exits the mouth
Platyhelminthes - Osmoregulation and Excretion
- Protonephridia present, key to invasion of freshwater and terrestrial envs.
- Act primarily for osmoregulation
Flame bulbs -
- cilia in mesh cup
- Beating cilia looks like a flickering flame
Drain into networks of collecting tubules called
excretory ducts or canals
Platyhelminthes - Regenerative Capabilities
- Asexual fission – common in free-living species
- Neoblasts – totipotent stem cells, make up 20% of cell pop in adults, can become or renew any cell type
Cohort Cestoda -
- “tapeworms”
- Scolex – attachment to host
- Proglottids – reproductive segments of the body, 50,000 eggs
- Matures with age, embryos fill near posterior end
- Proglottids break free or burst open and release embryos in host feces
- Syncytial external covering called tegument with many folds
- No digestive tract!
- Obtain host fluids through pinocytosis or diffusion
Cohort Trematoda -
- “flukes”
- Lack external cilia, rely on musculature to move or ride along host bodily fluids
- Also has a thick outer tegument
- Uses oral sucker and acetabulum (ventral sucker) to attach to host
- Feed on host fluids, tissues or gut content
Mouth leads to esophagus and then ceca (gut)
Schistosoma mansoni
- Blood flukes – live in blood vessels near intestines
- 5 species infect 240 million people with schistosomiasis, or snail fever
Nemertea
- ribbon worms
- Coelomate, unsegmented, flattened dorsoventrally
- Complete gut w/ protonephridia
- Unique proboscis – snapomorphy!
- Closed circulatory system, some w/ hemoglobin
- Mostly gonochoristic with spiral cleavage
Nemertea - Body Plan
- Superficially resemble flatworms, with features of acoelomate and coelomates
- Share parenchyma, external cilia and mucus and protonephridia
- Centralized nervous system with ladder-like arrangement of nerves
Nemertea - Circulatory system
- They have a closed circulatory system
1. Blood flows through vessels – modified coelomic spaces
2. Blood flows bidirectionally
3. Some species have hemoglobin- oxygen carrying pigment of blood
4. No true heart
Nemertea - Feeding and Digestion
- Active and specific carnivorous hunters
- They have complete gut
Proboscis –
extensible tube floating in fluid-filled space called rhynchocoel
Nemertea - Reproduction
- Most are gonochoristic with external fertilization
- After fertilization, cleavage is spiral and determinate
Pilidium larvae –
- planktotrophic, give rise to juveniles through indirect development
- then they either swim off and die or get ingested by the young after metamorphosis
- Only in Subclass Pilidophora
- Undergo fragmentation
