Porifera, Cnidaria, Platyhelminthes, & Nematoda

Explanation
Cnidaria is the correct answer because this phylum is known for its radial symmetry. Radial symmetry means that the body of an organism can be divided into equal halves in multiple planes, with each half mirroring the other. Cnidarians, such as jellyfish and sea anemones, exhibit this type of symmetry, with their body parts arranged around a central axis. In contrast, Nematoda, Platyhelminthes, and Porifera do not display radial symmetry.

Explanation
The correct answer is Porifera. Porifera is the phylum that includes sponges. Sponges are multicellular organisms that lack true tissues and organs. They have a porous body structure with numerous pores and channels that allow water to flow through. Sponges are filter feeders, meaning they extract food particles from the water that passes through their bodies. They are found in both freshwater and marine environments and play important roles in their ecosystems.

Explanation
Platyhelminthes is the correct answer because it is the phylum commonly known as flatworms. This phylum includes various species of worms that have a flattened body shape. They are characterized by their soft, unsegmented bodies and lack of a body cavity. Platyhelminthes encompass a diverse group of organisms, including tapeworms, flukes, and planarians.

Explanation
Nematoda is the correct answer because it is the first phylum to have a one-way digestive system. This means that food enters through one end of the digestive tract and exits through the other end, allowing for a more efficient digestion process. Cnidaria, Platyhelminthes, and Porifera do not have a one-way digestive system, making Nematoda the correct choice.

Explanation
Jellyfish belong to the phylum Cnidaria. This phylum includes a diverse group of animals that have specialized cells called cnidocytes, which they use to capture prey and defend themselves. Jellyfish have a gelatinous body and typically have tentacles with stinging cells. They are found in marine environments worldwide and play important roles in marine ecosystems.

Explanation
Cnidocytes are the specialized cells found in cnidarians, such as jellyfish and corals, that are responsible for delivering a sting. These cells contain a structure called a nematocyst, which is a coiled thread-like tube that is ejected when triggered. The nematocyst injects venom into the prey or predator, immobilizing or killing it. Cnidocytes are found in both the medusa (bell-shaped) and polyp (tube-shaped) forms of cnidarians, making them an essential defense mechanism for these organisms.

Explanation
Medusa is the correct answer because it refers to the free-swimming form of cnidarians. Cnidarians, such as jellyfish, have two main body forms: the sessile polyp and the free-swimming medusa. The medusa form is characterized by a bell-shaped body with tentacles hanging down, allowing for movement in the water. This form is responsible for reproduction and dispersal of cnidarian species.

Explanation
Nematodes have a pseudocoelom, which is a body cavity that is not completely lined by mesoderm. This means that the organs in the body of nematodes are not completely surrounded by a fluid-filled cavity, but rather are suspended in a fluid-filled space that is partially lined by mesoderm. This type of body cavity provides some protection and support for the organs, but it is not as well-developed as a true coelom, which is a body cavity that is completely lined by mesoderm.

Explanation
Spicules are tiny spike-like structures found in porifera, which are also known as sponges. These structures provide support and protection to the sponge. They are made up of various materials such as calcium carbonate or silica. Spicules can come in different shapes and sizes, depending on the species of sponge. They play a crucial role in maintaining the structural integrity of the sponge and also help in defense against predators.