A planula larva is the ciliated, free-swimming larval stage of a cnidarian. In General Biology I, it is the stage that leaves the fertilized egg, disperses through water, and eventually settles to become a polyp.
In General Biology I, a planula larva is the early, free-swimming stage of many cnidarians after fertilization. It is usually small, ciliated, and shaped to move through water until it finds a place to attach. Once it settles on a suitable surface, it changes form through metamorphosis and becomes a polyp.
The cilia are the main feature that make the planula work. Instead of using muscles for strong swimming, the larva beats its cilia to drift and travel in the water column. That movement gives the organism a way to spread away from the parent and look for a new habitat, which matters a lot for animals like corals, jellyfish, and sea anemones.
The planula is part of the cnidarian life cycle, so it sits between fertilization and the juvenile or adult body form. In species with a polyp stage, the planula settles and attaches to a surface such as rock or the ocean floor. After attachment, the larva reorganizes its body tissues and becomes a sessile polyp, which is the life stage that can later reproduce or bud off other forms depending on the species.
This stage is easy to misunderstand if you think of larval life as just a smaller version of the adult. A planula is not a tiny jellyfish or a tiny coral polyp. It is a temporary developmental form with a specific job: disperse, survive in the water, and find the right place to settle.
Its shape can vary across cnidarian groups, but the basic pattern stays the same. A fertilized egg develops into a mobile larva, the larva swims away from the parent, and then it switches to a settled life as a polyp. That change from movement to attachment is one of the clearest examples of life cycle stage specialization in animal biology.
The planula larva shows how cnidarians combine reproduction, dispersal, and development in one life cycle. If you are tracing how a cnidarian population spreads, the planula is the stage that explains how offspring leave the parent area and colonize new space instead of all staying crowded in one spot.
It also connects directly to the bigger cnidarian body plan. A planula turns into a polyp, and that shift helps you see that cnidarians can have very different-looking stages within the same organism. That matters when you compare forms like polyp and medusa, or when you study metagenesis in species that alternate between body types.
In lab or class discussion, planula larvae are a good example of form matching function. Cilia, free swimming, settlement, and metamorphosis each serve a separate step in the cycle. If you can explain why the larva swims first and attaches later, you are really explaining how cnidarians survive, disperse, and reproduce across different environments.
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Visual cheatsheet
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The planula larva settles and metamorphoses into a polyp in many cnidarians. That makes the polyp the next major body form in the life cycle, usually the sessile stage that attaches to a surface. If you are tracing development, the planula is the mobile precursor and the polyp is the attached outcome.
Metagenesis
In cnidarians with alternating body forms, the planula fits into the larger pattern of metagenesis. The larva is not the alternate adult form itself, but the stage that bridges reproduction and the sessile polyp phase. It helps show how one life cycle can include both attached and free-moving stages.
Radial symmetry
Cnidarians are known for radial symmetry, and the planula is part of that animal group. Even though the larva is moving and temporary, it still belongs to a lineage whose body plan is organized around a central axis. Seeing the planula in context helps you connect development with the adult cnidarian body plan.
Cnidocytes
Planula larvae belong to cnidarians, the same phylum that later develops cnidocytes in many species. The larval stage itself is mainly about movement and settlement, not stinging prey. This distinction matters because it keeps you from assuming every cnidarian stage has the same structures or job.
A quiz question might show a life cycle diagram and ask you to identify the stage that swims before the polyp forms. You would point to the planula larva and explain that it is ciliated, free-swimming, and used for dispersal. If a short answer asks how cnidarians colonize new habitat, the planula is the stage you describe first.
In a lab image, look for a small oval or pear-shaped larva covered in cilia rather than tentacles or a fixed body plan. On essays or discussion prompts about cnidarian diversity, you can use the planula to show that development in animals often includes a temporary stage with a specialized function before the adult form appears.
Planula larva and polyp are often confused because both are part of the cnidarian life cycle. The planula is the swimming larval stage that disperses and then settles, while the polyp is the attached body form that grows after settlement.
A planula larva is the free-swimming, ciliated larval stage of many cnidarians.
Its job is to disperse through water, then settle on a suitable surface and metamorphose into a polyp.
The cilia are what let the larva move, so mobility is built into its structure.
The planula is a temporary developmental stage, not a tiny adult cnidarian.
If you are tracing a cnidarian life cycle, the planula comes after fertilization and before the sessile polyp stage.
A planula larva is the ciliated, free-swimming larval stage of a cnidarian. It develops from the fertilized egg, moves through the water, and later settles to become a polyp.
It moves with cilia covering its surface. The cilia beat to help it swim and drift until it finds a place to attach.
In many cnidarians, the planula larva settles on a substrate and undergoes metamorphosis into a polyp. That is the attached stage that follows larval dispersal.
No. A planula is the mobile larval stage, while a polyp is the settled body form that comes after attachment. They are connected stages, but they do different jobs in the life cycle.