Question

Cnidarians alone have _____. a. cnidocytes c. a hydrostatic skeleton b. a mantle d. a radula

Short answer

Cnidarians alone have cnidocytes.

Step-by-step solution

Understanding Cnidarians

Cnidarians are a group of animals that include jellyfish, coral, sea anemones, and hydra. They are known for their simple body structure, which includes radial symmetry and a single opening serving as both mouth and anus.

Defining Cnidocytes

Cnidocytes are specialized cells unique to cnidarians. These cells contain organelles called nematocysts, which are capable of delivering a sting to prey or predators. This is a defining feature of cnidarians.

Examining Other Options

- A hydrostatic skeleton is a feature found in many invertebrates, not exclusive to cnidarians. - A mantle is a part of mollusks, not cnidarians. - A radula is a structure found in mollusks, specifically used for feeding, and is not present in cnidarians.

Conclusion

Given that cnidocytes are the unique characteristic of cnidarians, the correct answer to the question is 'cnidocytes.'

Key concepts

Cnidocytes

Cnidocytes are specialized cells unique to the Cnidarian family. These cells serve a critical function for cnidarians, acting primarily as a defense mechanism and as a means of capturing prey. Within each cnidocyte, there is a specialized subcellular structure called a nematocyst.
Notably, cnidocytes distinguish cnidarians from other invertebrate groups.
When stimulated, these cells eject a coiled thread that can penetrate, entangle, or immobilize prey. This unique mechanism is often compared to a tiny harpoon or needle.

• Function: Cnidocytes help in prey capture and protection.
• Location: Found on the tentacles or outer surface.
• Trigger: Chemical or physical stimulation causes discharge.

Cnidocytes are essential for survival, as they compensate for the creatures' otherwise simple structure.

Radial Symmetry

Radial symmetry is a basic body plan found in cnidarians. This symmetry allows the organism to interact with its environment from all angles, which is particularly advantageous in aquatic habitats. Essentially, if you cut a radially symmetrical organism any way through its central axis, all resulting halves are symmetrical.
Radial symmetry contrasts with bilateral symmetry, where only one plane splits the body into mirrored halves.

• Evolutionary Advantage: Enables similar interactions in all directions.
• Structure: Cnidarians like jellyfish have multiple radiating parts.
• Immediate Response: Efficiently responds to stimuli from any direction.

This form of symmetry helps cnidarians function effectively, despite their simple organization.

Nematocysts

Nematocysts are the stinging organelles contained within cnidocytes. These structures are nano-sized weapons that cnidarians use to inject toxins into their targets.
This sharp thread with a barbed tip can launch in microseconds and for its size, one of the fastest biological mechanisms.
This feature is crucial for both feeding and protection of cnidarians like jellyfish and sea anemones.

• Activation: Nematocysts are triggered under contact or chemical cues.
• Components: Includes a capsule, coiled thread, and toxins.
• Purpose: Used to paralyze prey and deter predators.

The venom delivered by nematocysts varies among species, influencing the intensity of their sting.

Invertebrates

Cnidarians are part of a larger group called invertebrates, a category that encompasses animals lacking a vertebral column. Invertebrates make up the vast majority of animal diversity. They exhibit incredible variety in both form and function, inhabiting numerous ecosystems and playing essential roles in their environments.
Some characteristics shared by many invertebrates include simple body structures and diverse reproductive tactics.

• Categories: Includes other animals like mollusks and arthropods.
• Complexity: Invertebrates range from simple organisms to complex beings.
• Role: Serve key ecological roles in the food chain and nutrient recycling.

Cnidarians, despite their simplicity, are an integral part of the marine invertebrate community.