Hint: In Hydra, a cnidarian, the cnidocytes contain specialized structures called nematocysts, which contain poison. Nematocysts are a coiled hollow thread that is triggered by cnidocil. Upon contact with prey, the contents of the nematocyst are explosively discharged, which can paralyze the prey for defense.
Step by step answer:Hydras are tiny, simple invertebrates commonly studied by beginning biology students. They belong to the phylum Cnidaria, which includes corals, sea anemones, and jellyfish. Unlike most cnidarians, which are marine animals, hydras are found in freshwater ponds, lakes, and streams. Hydras are also typical because they do not have a medusa (jellyfish) stage as part of their life cycle as do most other cnidarians. They live and reproduce sexually and asexually, but only in the tube-shaped polyp form. However, they do have nematocysts, or cnidae, the microscopic intracellular stinging capsules characteristic of this phylum and for which it is named. Simple as these organisms are, the nematocysts are one of the most complex structures in the animal world.
Nematocysts are produced by nematoblasts, or cnidoblasts, and are used for capturing and paralyzing prey or for defense. Each nematocyst contains a coiled, hollow thread that can have barbs or spines and often contains poison. Under the appropriate stimulation, it is everted from the capsule, poisoning, or entangling the organism’s victim. Stings from large jellyfish, such as the Portuguese man-of-war can be extremely painful to humans and may cause paralysis, shock, or even death.
Hydras have four types of nematocysts on their tentacles and ectoderm (outer layer of cells), which are used for a variety of purposes. The largest nematocyst has barbs on the end that anchor the prey to the tentacle from which it was fired. With a firm hold on its prey, the hydra then envelopes the organism, like a sock being pulled over a foot, and consumes it. The second type is smaller and has a shorter, thicker corkscrew thread that wraps around and holds onto the prey animal. A third type has a sticky bean-shaped object at its end that is used in locomotion, securing the hydra as it glides or somersaults from one place to another. The fourth kind of nematocyst has spikes running along the thread and is probably used to defend the hydra against potential predators.
Nematocysts are a type of cnidae, and it is the presence of cnidae that separates jellyfish and other cnidarians from other animals. Cnidae are among the most complex intracellular secretion products known. If a toxin is present, it passes through the hollow thread, penetrating and paralyzing the victim’s tissues. After eversion, the thread separates from the nematocyst. The threads of some nematocysts ensnare small prey by wrapping about them.
The discharge of the nematocyst thread tube from the cnidocyte takes place due to mechanical or chemical stimuli received by the cnidocil. In a resting stage, the capsule wall is not permeable to water.
And there is very high osmotic pressure since the hypnotoxin inside is hypertonic to external water. As the cnidocil is triggered, the permeability of the capsule wall increases causing water rush inside the capsule, resulting in increased hydrostatic pressure.
Hence option B is correct.
Note: All coelenterates are aquatic, mostly marine. The body-form is radially symmetrical, diploblastic and does not have a coelom. The body has a single opening, the hypostome, surrounded by sensory tentacles equipped with either nematocysts or colloblasts to capture mostly planktonic prey.