Examkin

Question

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

An animal specimen collected during a field study is triploblastic and bilaterally symmetrical, featuring a fluid-filled body cavity that is not completely lined by mesoderm (pseudocoelom) and a complete digestive tract with both a mouth and an anus. Which phylum does this organism belong to?

  1. NematodaAnswer
  2. B
    Platyhelminthes
  3. C
    Coelenterata
  4. D
    Porifera

Answer

Nematoda
Nematoda is correct because members of this phylum (roundworms) are triploblastic, bilaterally symmetrical, pseudocoelomate (having a body cavity derived from the blastocoel that is not lined by peritoneum), and possess a complete digestive system with separate mouth and anus.

Step-by-Step Solution

1
Analyze the germ layers and body symmetry described in the question stem.
The organism is triploblastic and bilaterally symmetrical, which eliminates Porifera (cellular level, no true germ layers) and Coelenterata (diploblastic, radially symmetrical).
Lower invertebrates show progressive tissue organization from cellular (Porifera) to diploblastic (Coelenterata) to triploblastic bilateral structures (Platyhelminthes and Nematoda).
2
Evaluate the body cavity (coelom) type and digestive tract structure.
The presence of a pseudocoelom (body cavity not completely lined by mesoderm) and a complete digestive tract with separate mouth and anus points specifically to Phylum Nematoda.
Platyhelminthes are acoelomate with an incomplete gut, whereas Nematoda are pseudocoelomate with a complete gut.

Key Concept

Diagnostic structural characteristics (symmetry, germ layers, coelom, and gut completeness) of Phylum Nematoda
Rate this question

Topics

Lower Invertebrates: Porifera, Coelenterata, Platyhelminthes, and Nematoda