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Question

Difficulty: MediumLower Invertebrates: Porifera, Coelenterata, Platyhelminthes, and Nematoda

Lower invertebrates display distinct evolutionary milestones in body symmetry, tissue organization, and body cavity structure. Match each lower invertebrate phylum on the left with its corresponding diagnostic anatomical characteristic on the right.

  • PoriferaCellular level of organization with body wall perforated by ostia and lined with choanocytes
  • CoelenterataDiploblastic body structure with radial symmetry and specialized nematocysts
  • PlatyhelminthesTriploblastic acoelomate body plan with flame cells for osmoregulation
  • NematodaTriploblastic pseudocoelomate body plan with an unsegmented body and complete gut

Answer

Porifera corresponds to cellular organization with choanocytes and ostia; Coelenterata corresponds to diploblastic radial symmetry with nematocysts; Platyhelminthes corresponds to triploblastic acoelomate structure with flame cells; Nematoda corresponds to triploblastic pseudocoelomate structure with a complete digestive tract.
Each taxon is uniquely identified by its evolutionary grade: Porifera exist at the cellular level with choanocytes; Coelenterata display diploblastic tissue organization with nematocysts; Platyhelminthes are triploblastic acoelomates with flame cells; Nematoda are triploblastic pseudocoelomates with a complete digestive system.

Step-by-Step Solution

1
Identify the organizational and cellular hallmarks of Porifera.
Porifera are sponges with no true tissues, relying on ostia for water entry and choanocytes to generate water currents.
This cellular-level plan distinguishes Porifera from tissue- and organ-grade invertebrates.
2
Analyze the germ layer arrangement and specialized defense cells of Coelenterata.
Coelenterata have two germ layers (diploblastic), radial symmetry, and possess nematocysts (cnidocytes).
Nematocysts are unique stinging structures exclusive to Cnidarians/Coelenterates.
3
Determine the coelom status and excretory organ of Platyhelminthes.
Platyhelminthes are triploblastic flatworms lacking a body cavity (acoelomate) and using flame cells for osmoregulation.
The absence of a coelom combined with protonephridia defines Platyhelminthes.
4
Examine the body cavity type and alimentary canal of Nematoda.
Nematoda are roundworms featuring a persistent blastocoel (pseudocoelom) and a complete mouth-to-anus gut.
The pseudocoelom provides hydrostatic support and surrounds an alimentary canal with separate openings.

Key Concept

Structural organization, germ layer arrangement, coelom type, and diagnostic cell/tissue types across lower invertebrate phyla
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Topics

Lower Invertebrates: Porifera, Coelenterata, Platyhelminthes, and Nematoda