Invertebrates Lecture 2 Notes - Platyhelminthes, molluscs and annelids
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Course
University Of Leicester (BS1070)
Institution
University Of Leicester (LE)
Detailed and concise FIRST CLASS notes which are easy to understand and learn from, covering the second lecture of Invertebrate Zoology in the BS1070 module.
University of Leicester - BS1070 - Week 4 - Adaption and diversity
Invertebrates - Lecture 2: Platyhelminthes, Molluscs and Annelids
Bilaterian phylogeny
- Bilaterally symmetrical (one symmetrical axis) organisms with a mesoderm (triploblastic)
- Traditional bilateral phyla are defined by cleavage patterns, mesoderm formation and coelom
- These do not necessarily reflect phylogenetic relationships
- Molecular phylogeny is based on sequence similarities (e.g. 18S ribosomal RNA)
- Indicates relatedness
- Phylogenetics determined protostomes and deuterostomes
- Protostome: mouth forms before the anus e.g. flatworms, annelids, arthropods
- Deuterostomes: mouth forms after the anus e.g. humans, tigers, monkeys
Lophotrochozoa
One of two main protostome clades
- Very diverse assemblage of morphologically disparate phyla
- Typically with spiral cleavage
- Unites two major groupings (clades)
- Lophophorata
- Trochozoa
- And the platyhelminthes
Platyhelminthes
Bilateria without a coelom - 20 000 species today
Planarians
- Free living non-parasitic
flatworms
- Mostly predators, creep
with ciliated epidermis
(smaller) OR crawl with
muscular wave (larger)
- Have a head with
eye-spots and brain
- Have a mouth on ventral
surface, half way down
the belly side
- Flatworms have bilateral symmetry
- Have a distinct head and tail end
- Distinct dorsal and ventral surfaces
- They are mesodermal, therefore triploblastic
- Have tissues and organs → CNS: brain and longitudinal nerve cords
, University of Leicester - BS1070 - Week 4 - Adaption and diversity
- Planarians do not have an anus
- Eat and excrete out of their mouth
- Acoelomates: no body cavity
- Mesodermal mesenchymal fills entire space between endoderm and ectoderm
- No circulatory or respiratory organs
- Simple diffusion of gases through their body surface
- Thin and long, increases surface area
- Gut forms a gastrovascular system
Planarians - reproduction
- Sexual hermaphrodites (male and female gametes)
- Also asexually reproduce via spontaneous fissions
- Experiment and accidental regeneration
- Can be cut up lengthwise or across, each piece will degenerate into the complete animal
Parasitic platyhelminthes (endoparasites)
- Trematoda (flukes): tissue parasites - blood, liver and lungs
- Cestoda (tapeworms): gut parasites
- Consequences of living indies the gits of another organism:
- Needs for resistant covering to protect against host’s digestive enzymes → loss of cilia
- Surrounded by food, no directional external threats → reduction of nervous system, loss
of cephalisation
- Don’t need a gut for yourself → loss of gut, direct absorption through microvilli on body
surface
Lophophorata
- Filter feeders
- With a lophophore → horse shoe shaped feeding organ of ciliated tentacles around the mouth
- Have four clades (considered phyla)
- Bryozoa: ‘moss animals’
- Entoprocta/kamptozoa
- Phoronida
- Brachipoda: ‘lamp shells’
Trochozoa
- Typically develop through a planktonic trochophore
larva
- Include mollusca, annelida and nemertea
-
Form of trochozoa often found in aquatic animals.
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