Cnidaria: Characteristics & Hydra | Biology for JAMB PDF Download

Coelenterates, also known as Cnidaria, are a diverse group of aquatic animals that include organisms such as jellyfish, sea anemones, and corals. They exhibit certain general characteristics that distinguish them from other animal groups. Here are some key points to note about Cnidaria:

General Characteristics of Cnidaria

• Coelenterates are exclusively aquatic organisms, found in both marine and freshwater environments.
• They possess a radially symmetrical body plan, which means their body parts are arranged around a central axis.
• Cnidarians have a diploblastic organization, meaning their body is composed of two primary layers: an outer ectoderm and an inner endoderm, separated by a gelatinous layer called the mesoglea.
• They possess specialized stinging cells called cnidocytes, which contain nematocysts. These cells help in capturing prey and defense.
• The body of a Cnidarian can exist in two basic forms: a polyp form (sessile and cylindrical) and a medusa form (bell-shaped and free-swimming).
• Cnidarians have a simple nervous system composed of nerve nets and sensory structures that allow them to detect and respond to stimuli.
• They exhibit a gastrovascular cavity, which functions in both digestion and circulation.
• Cnidarians lack specialized respiratory or excretory organs.

Hydra

• Hydra is a common freshwater coelenterate that represents the polyp form of Cnidaria.
• Structure: Hydra has a tubular body with a cylindrical shape and is attached to a substrate at its base. The body is composed of an outer layer called the epidermis and an inner layer called the gastrodermis, with a mesoglea layer in between.
• Reproduction: Hydra reproduces both sexually and asexually. Asexual reproduction occurs by budding, where small outgrowths called buds develop from the body wall and eventually separate to form new individuals. Sexual reproduction occurs by the fusion of male and female gametes to produce zygotes.
• Nutrition: Hydra is a carnivorous organism. It captures small aquatic organisms, such as small crustaceans and insects, using its tentacles armed with stinging cells. When prey comes in contact with the tentacles, the cnidocytes discharge nematocysts to paralyze and immobilize the prey. The tentacles then bring the captured prey to the mouth, located at the center of the body, for ingestion.
• Feeding Mechanism: Hydra uses its gastrovascular cavity for both digestion and distribution of nutrients. It secretes enzymes into the cavity to break down the prey, and the resulting nutrients are absorbed by the cells lining the gastrodermis.
• Respiration and Excretion: Hydra obtains oxygen and eliminates waste products through simple diffusion across the body surface since it lacks specialized respiratory or excretory structures.
• Sensory and Nervous System: Hydra possesses sensory cells, especially on its tentacles, which help in detecting prey and other stimuli in the environment. It also has a simple nervous system consisting of nerve nets that allow coordination of movement and responses to external stimuli.