Phylum Cnidaria | Animal kingdom | Biology | Khan Academy

Summary

This video explores the fascinating world of jellyfish, focusing on their biology and unique features. Jellyfish, belonging to the phylum *Cnidaria*, are marine animals known for their radial symmetry and two body forms: the Medusa (umbrella-shaped) and Polyp (fixed). The script delves into their internal systems, including the gastrovascular cavity, which functions both as a digestive and circulatory system. Key to their survival are nematocysts, stinging cells used for offense, defense, and attachment. The video also covers their life cycle, highlighting alternation of generations between polyp and medusa forms, with both sexual and asexual reproduction methods.

Takeaways

• 😀 Jellyfish belong to the phylum Cnidaria, which is known for its stinging cells called nematocysts.
• 😀 These marine creatures have a graceful swimming appearance but can deliver painful stings, with some species being deadly.
• 😀 Jellyfish are diploblastic, meaning they have two tissue layers: ectoderm (outer) and endoderm (inner).
• 😀 The body of jellyfish exhibits radial symmetry, meaning it can be divided into equal parts along multiple planes.
• 😀 Jellyfish have two primary body forms: the Medusa (umbrella-shaped, free-swimming) and the Polyp (fixed to surfaces).
• 😀 The gastrovascular cavity in jellyfish serves both as their digestive and circulatory system.
• 😀 Digestion in jellyfish is both intracellular (within cells) and extracellular (within the gastrovascular cavity).
• 😀 Nematocysts are specialized cells in jellyfish that sting, used for offense, defense, and attachment to surfaces.
• 😀 Jellyfish undergo alternation of generations, transitioning between asexual reproduction (Polyp to Medusa) and sexual reproduction (Medusa to Polyp).
• 😀 The polyp form is fixed to a surface, while the medusa form is free-swimming and responsible for reproduction in jellyfish.
• 😀 In sexual reproduction, Medusas release sperm and eggs, which fertilize to form a larva that eventually becomes a polyp.

Q & A

What are the two main body forms found in Cnidarians?

The two main body forms found in Cnidarians are the Medusa and the Polyp. The Medusa has a bell-shaped, umbrella-like structure, while the Polyp is usually cylindrical and fixed to a surface.

What is the function of the gastrovascular cavity in Cnidarians?

The gastrovascular cavity serves as both the digestive and circulatory system. It processes food, absorbs nutrients, and circulates water, oxygen, and carbon dioxide throughout the organism.

What is the primary function of the nematocyst in jellyfish and other Cnidarians?

The nematocyst is a stinging structure within a cnidocyte (stinging cell). It is used for offense (to capture prey), defense (to ward off predators), and attachment (to help the organism adhere to surfaces).

What type of symmetry do jellyfish exhibit?

Jellyfish exhibit radial symmetry, meaning their body can be divided into multiple equal parts by several planes through the central axis.

How do Cnidarians perform digestion?

Cnidarians perform digestion through both extracellular and intracellular digestion. Food is ingested through the mouth into the gastrovascular cavity, where some digestion occurs outside cells, while some happens inside the cells.

How does the alternation of generations work in Cnidarians?

Alternation of generations in Cnidarians involves the switching between the Medusa and Polyp forms. In species with both forms, asexual reproduction occurs when the Polyp buds off a Medusa, while sexual reproduction involves the fertilization of eggs and sperm released by Medusas.

What is the significance of the term 'diploblastic' when referring to Cnidarians?

Diploblastic refers to the fact that Cnidarians have two embryonic tissue layers: the ectoderm (outer layer) and the endoderm (inner layer), which surround the gastrovascular cavity.

What role does water play in the physiology of Cnidarians?

Water serves as a medium for both nutrient and oxygen intake, as well as the expulsion of waste and carbon dioxide. It enters through the mouth and facilitates the function of the gastrovascular cavity, acting as both a digestive and circulatory medium.

Why are corals important in understanding Cnidarians?

Corals are important because they have a hard skeleton made of calcium carbonate, which is part of their structural support. They also serve as a crucial part of marine ecosystems, often forming coral reefs that support diverse marine life.

What happens when a Cnidarian's nematocyst is triggered?

When a nematocyst is triggered, it rapidly uncoils, and its barbed thread penetrates the skin of a prey or predator, injecting a toxin that can paralyze or kill the target.