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Questions and Answers
What is a notable characteristic of Placozoa?
What is a notable characteristic of Placozoa?
How do Placozoa primarily reproduce asexually?
How do Placozoa primarily reproduce asexually?
What structure do Ctenophores use for prey capture?
What structure do Ctenophores use for prey capture?
Which of the following statements about Ctenophora is true?
Which of the following statements about Ctenophora is true?
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What is unique about the musculature of Ctenophores?
What is unique about the musculature of Ctenophores?
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In Ctenophores, how are gametes typically released?
In Ctenophores, how are gametes typically released?
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What type of symmetry do Placozoans exhibit?
What type of symmetry do Placozoans exhibit?
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Which of the following best describes the nervous system of Ctenophores?
Which of the following best describes the nervous system of Ctenophores?
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Study Notes
Phyla Placozoa & Ctenophora
- Placozoa and Ctenophora are invertebrate phyla.
- Placozoa are minute, flattened metazoans.
- They are composed of ciliated upper and lower cell layers.
- Ameboid and fibrous cells are in a gel-like layer between the cell layers.
- Adults are discoidal and asymmetrical or have polytomous branching.
- They have six somatic cell types.
- Cells have desmosome cell-cell connections.
- They possess unique shiny spheres in the upper cell layer, possibly defensive structures.
- They lack a nervous system, muscles, or digestive system.
Placozoa Feeding
- Placozoa move toward a nutrient particle using cilia.
- They form a digestive cavity around the particle and digest it externally.
Placozoa Reproduction
- Asexual reproduction: Fission and swarmer formation (budding yields multicellular flagellated swarmers).
- Sexual reproduction is known to occur but not well studied.
Ctenophora
- Ctenophora are also invertebrates, and are related to Placozoa.
- ~130 species, known as comb jellies, sea gooseberries, or sea walnuts
- Predominantly small, but some can grow up to a meter.
- Exclusively marine, mostly pelagic.
- Predatory, trapping prey with colloblast cells on tentacles.
Ctenophora Feeding & Transport
- Ctenophores have highly branched gastrovascular canals throughout the body.
- Lack dedicated respiratory and circulatory systems.
- Gastrovascular system distributes nutrients and likely removes waste.
Ctenophore Characteristics
- Diploblastic (or possibly triploblastic) metazoa.
- Ectoderm and endoderm separated by a cellular mesenchyme (mesenchyme is endodermal in origin).
- Biradial symmetry with oral-aboral axis.
- Exocytotic structures (colloblasts).
- Gastrovascular cavity (gut) is the only body cavity, with a stomodeum and canals branching throughout the body; gut ends in two small anal pores.
- Lack discrete respiratory, excretory, or circulatory systems.
- Nervous system is a nerve net or plexus (more specialized than cnidarians), some species with giant axons.
- Striated muscles (true mesenchymal cells).
- Eight rows of ciliary plates (ctenes) at some stage in their life history.
- Comb rows controlled by unique apical sense organ.
- Some adults and most juveniles have a pair of tentacles (often retractable).
- Monomorphic, lacking alternation of generations and sessile life stages.
- Hermaphroditic, typically with a characteristic cydippid larval stage.
Ctenophore Colloblasts
- Multicellular structures used for prey capture.
- Produce sticky “glue” to capture prey.
Apical Sensory Organ
- Used to sense orientation and gravity.
- Simple net-like nervous system.
Ctenophore Reproduction & Development
- High regenerative ability; possible asexual reproduction through damage or fragmentation.
- Most species are hermaphroditic, others are gonochoristic, with external fertilization (but some internal fertilization).
- Diploblastic (maybe triploblastic) development.
Evolutionary Relationships
- Phylogenetic relationships between Porifera, Cnidaria, Bilateria, Placozoa, and Ctenophora are subject to ongoing research and debate.
- Some studies show Porifera as sister to the remaining phyla.
- Other studies suggest Ctenophora are closer to other phyla and that Placozoa are a sister group to Ctenophores or Cnidarians or Bilateria.
Ctenophore Musculature
- Possess radial and looped fibers and meridional and latitudinal fibers.
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Description
Explore the fascinating characteristics of the invertebrate phyla Placozoa and Ctenophora. Learn about their structure, feeding mechanisms, reproductive strategies, and unique features that distinguish them from other organisms. This quiz will deepen your understanding of these lesser-known but intriguing marine animals.
