This lesson introduces the defining features of cnidarians and the two body forms found in the phylum.
Cnidarians are animals with radial or biradial symmetry and diploblastic development. Diploblastic animals develop from two embryonic layers: the ectoderm and endoderm. About 99 percent of cnidarian species are marine.
The phylum’s defining cell is the cnidocyte, a specialized stinging cell found especially around the mouth and on the tentacles. Cnidocytes help capture prey and defend against predators.
All flashcards
What does diploblastic mean?
An animal is diploblastic when it develops from two embryonic layers: ectoderm and endoderm.
What is a cnidocyte?
A cnidocyte is a specialized cnidarian stinging cell, commonly found on tentacles and around the mouth, that helps capture prey or repel predators.
How does a nematocyst work?
When triggered, its coiled hollow thread rapidly everts, entangling or penetrating a target and sometimes delivering toxins.
How do polyp and medusa forms differ?
A polyp is usually attached, with its mouth facing upward; a medusa is usually free-swimming, with its mouth and tentacles hanging down from a bell.
What sequence links the polyp and medusa stages in many dimorphic cnidarians?
Polyps commonly reproduce asexually to produce medusae; medusae produce gametes, and fertilization leads to a planula that settles and develops into a polyp.
Quiz questions
Which two embryonic layers develop in a diploblastic cnidarian?Answer: Ectoderm and endoderm.
Which specialized cell type defines the phylum Cnidaria?Answer: A cnidocyte, a specialized stinging cell.
What role does the cnidocil play in a cnidocyte?Answer: It detects tactile stimulation that can trigger the nematocyst.
A cnidarian is attached to a surface and has an upward-facing mouth surrounded by tentacles. Which body form is it?Answer: A polyp.
After a cnidarian planula larva settles on a surface, what does it typically develop into?Answer: A polyp.
Based on Biology 2e by Mary Ann Clark, Matthew Douglas, Jung Choi (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.