Answer:

Introduction:
Metamorphosis is a biological process in which an animal undergoes significant changes in its body structure and physiology as it progresses through different stages of its life cycle. It is a common phenomenon observed in many animals, particularly insects and amphibians. However, not all animals undergo metamorphosis.

Explanation:
Among the given options, the animal that does not undergo metamorphosis is the earthworm. Earthworms belong to a group of animals known as annelids and they exhibit a type of development called direct development.

Direct development:
Direct development is a type of development in which an animal hatches from an egg and grows into an adult without undergoing any significant changes in its body structure. In the case of earthworms, the young ones that hatch from the eggs look similar to the adults and do not go through any distinct larval stages or metamorphic changes.

Metamorphosis in other animals:
On the other hand, the other options given - tunicate, moth, and starfish - all undergo metamorphosis during their life cycle.

Tunicates:
Tunicates are marine animals that belong to the phylum Chordata. They undergo a type of metamorphosis known as biphasic metamorphosis. In this process, tunicates start their life cycle as free-swimming larvae called tadpole larvae. These larvae undergo various changes in their body structure and eventually settle on a substrate, where they undergo metamorphosis into an adult tunicate.

Moths:
Moths, along with butterflies, belong to the order Lepidoptera. They exhibit a type of metamorphosis called complete metamorphosis. The life cycle of a moth consists of four distinct stages: egg, larva (caterpillar), pupa (chrysalis), and adult. The larval stage is characterized by rapid growth and feeding, while the pupal stage is a non-feeding stage during which the larva undergoes dramatic changes inside a protective cocoon. Finally, the adult moth emerges from the pupa and is ready for reproduction.

Starfish:
Starfish, also known as sea stars, belong to a group of animals known as echinoderms. They undergo a type of metamorphosis called bipinnaria larvae. The larvae of starfish go through several stages of development before undergoing metamorphosis into the adult form. This metamorphosis involves the development of arms and tube feet, among other changes.

Conclusion:
In summary, among the given options, the animal that does not undergo metamorphosis is the earthworm. Earthworms exhibit direct development, while other animals like tunicates, moths, and starfish undergo various forms of metamorphosis during their life cycle.

A change in the form and often habits of an animal during normal development after the embryonic stage is called the metamorphosis.
While such a metamorphosis can be seen in moth, starfish, and tunicate, the earthworm does not undergo any kind of metamorphosis during its life cycle.