ANATOMY

Detailed explanation-1: -Nervous systems are found in almost all multicellular animals, but vary greatly in complexity. The only multicellular animals that have no nervous system at all are sponges and microscopic bloblike organisms called placozoans and mesozoans.

Detailed explanation-2: -Sponges are the only multicellular animals without a nervous system. They do not have any nerve cells or sensory cells. However, touch or pressure to the outside of a sponge will cause a local contraction of its body.

Detailed explanation-3: -Under the entry for “nervous system”, Encyclopædia Britannica states that [i]n animals, in addition to chemical regulation via the endocrine system (which plants have), there is another integrative system called the nervous system. Most dictionaries and textbooks maintain that only animals have nervous systems.

Detailed explanation-4: -Step by step answer:Hydra has anatomically one of the simplest nervous systems which is a nerve net formed by the nerve cells present in the epidermis of the Hydra with no sign of a brain.

Detailed explanation-5: -The nervous system is made up of the brain, spinal cord, and several different kinds of nerves that are found throughout the body. These create complex circuits through which animals experience and respond to sensations. A familiar type of nervous system circuit is a reflex.