Invertebrate Phyla: Placozoa & Ctenophora

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.