Lecture 20: Rouphozoa, Platyhelminthes

Rouphozoa Clade

• phyla platyhelminthes + gastrotricha

• characterized by ingestion via sucking

• duo-gland organs

  • 2-3 secretory organs for temporary adhesion

• acoelomates

Platyhelminthes

• nonmetameric worms

  • cestoda are strobilated, but not true segmentation

• includes flatworms, flukes and tapeworms

  • free living: turbellarians

  • parasitic: tremaatoda, monogenea, cestoda

Platyhelminthes Digestion

• complex but incomplete gut, which is absent in some parasitic forms

  • free living or parasitic

  • blind gut, meaning a mouth but no anus

• feed via extrusible muscular pharynx in ventral centre of body

• extracellular or intracellular digestion

Platyhelminthes Nervous System

• ladder like nervous system with anterior cerebral ganglia

  • chemoreceptors

    • auricles

    • ciliated pits

  • tactile receptors

  • rheoreceptors

  • statocysts

  • photoreceptors

• reduced nervous system in tapeworms (and other endoparasites)

Platyhelminthes Anatomy

• acoelomate

  • only one internal space: digestive cavity

  • region between epidermis and digestive cavity filled with parenchyma

  • no circulatory system or coelom

• bilaterally symmetrical

• dorsovenetrally flattened

• multiciliated epidermal cells

Platyhelminthes Excretion

• depend on a network of protonephridia for excretion and osmoregulation

  • flame cells + tube cells = protonephridia

• filters fluids from inside body

• removes metabolic wastes

• controls osmotic pressure

• retains important ions

Platyhelminthes Reproduction/Development

• tremendous capacity for regeneration and asexual reproduction

  • but retains polarity

• most are hermaphroditic, and have internal fertilization and direct development

  • if larval form is present: Müller’s larva (not trochophore)

• primitive forms show spiral cleavage

  • advanced forms have yolk-covered eggs

Platyhelminthes Skin/Muscle

• multiciliated cells produce mucus

• some syncytial

  • a single cell that contains multiple nuclei

• epidermis contains rhabdites

• attach-and-detach system made possible by dual-gland adhesive organs

Rhabdites

• rod-shaped, membrane-bound secretory organelles found in epidermis

• makes mucus for protection, gliding, etc.

Parasitic Platyhelminthes/Turbellarian similarities

• parasitic types share many traits with turbellarians

  • dorsoventrally flattened body

  • blind gut

  • protonephridia

  • hermaphroditic

Parasitic Platyhelminthes/Turbellarian dissimilarities

• parasitic types differ from turbellarians;

  • lack sense organs

  • epidermis lacks cilia

  • have syncytial epitheloum with microvilli

Parasitic Platyhelminthes Adaptations

• adults produce massive #s of gametes

• most have complex life cycles with 2+ hosts

  • final= definitive host, site of sexual reproduction

  • intermediate hosts= enhance transmission to definitive host, amplification of infective stages via asexual reproduction

Trematoda

• parasitic platyhelminthes flukes with 1-3 hosts

• adapted for parasitism via complex life cycle

• ex. Digenea sp.

  • requires 2 hosts (intermediate & definitive)

Digenean Fluke Life Cycle

1. intermediate host

   1. first host (mollusc), asexual amplification

   2. after amplification, a swimming form goes to infect vertebrate

2. definitive host

   1. final host, sexual amplification

   2. juvenille flukes can infect final host to mature and reproduce

   3. fertilized eggs in vertebrate feces

   4. eggs hatch in….. (back to top)

Common Digenean Flukes

• liver flkes

• blood flukes

  • ex. schistosomiasis (swimmers itch)

Monogenea Clade

• 1 host flukes

• on skins/gills of fish

  • ectoparasitic

  • opisthaptor attachment with many hooks

• simple direct life cycle

  • eggs → ciliated larvae → adult

Cestoda Clade

• tapeworms

• common gut parasites of all vertebrate classes

• needs at least 2 hosts

  • encyst in intermediate host

• long flat bodies with:

  • scolex

  • proglottids

  • no dig. system

  • microtriches

Scolex

• head attachment to host

  • acts as an anchor between worm to the intestinal epithelium

  • prevents cestoda from being passed with the digested food

• some cestodes carry a protrusible rostellum

  • armed with hooks

• includes usually 4 suckers per scolex

• no mouth, no gut

Proglottis

• reproductive units packed with infective larvae

• forms strobila

  • new proglottids are continuously formed in the neck just below the scolex

• length of worm and proglottids increase in size and maturity

  • developing from premature to mature to the gravid stage (at which point the prroglottid is filled with the uterus and eggs)

Strobila

• not true segmentation

  • independent segments, not working/coordinating together for the whole organism

• asexual formation of new segments (reproductive factories)

• youngest at head, mature at posterior

  • mature known as gravid

• eggs out of the uterine pore

Gravid

• most mature proglottids of a strobila

• found at the posterior end

Tapeworm Infection Life Cycle

• Cysticercus gets eaten by definitive host

• fertilized eggs in gravid proglottids

• protected larvae, out feces

• eggs ingested by intermediate host

• cysts in muscle

• cysticercus gets eaten by definitive host

Phylum Gastrotricha

• acoelomate

• eutely, microscopic free living bowling-pin shaped worms

Eutely

• biological condition where an adult organism consists of a fixed, specific number of somatic cells that is the same in all members of that species

Gastrotricha Skin

• bilayered cuticle

  • exocuticle covering entire body (including all cilia)

  • cuticle not molted

• duo-gland terminal adhesive tube projections

• epidemis cellular or partly syncytial

  • epidermal cells mono or multiciliate

  • locomotory cilia on ventral surface only

Gastrotricha Anatomy

• has a myoepithelial pharynx with tri-radiate lumen

• complete gut

• 1+ protonephridia

• no circulatory structures

• no gas exchange structures

Gastrotricha Reproduction/Development

• hermaphroditic, parthenogenetic, or both

• complex reproductive organs

• direct development