From an evolutionary perspective, the term "primitive" refers to a species that has retained more ancestral characteristics and has undergone less evolutionary change compared to others. In this question, we are asked to identify the most primitive animal among roundworm, spider, prawn, and snail.

Option 1: Roundworm - Roundworms, also known as nematodes, are considered to be among the most primitive animals. They are unsegmented and have a cylindrical body shape. Roundworms have a simple digestive system and lack complex organs such as respiratory or circulatory systems.

Option 2: Spider - Spiders belong to the class Arachnida and are not considered primitive in comparison to roundworms. They have evolved specialized features such as the ability to produce silk for web construction, venom for capturing prey, and complex respiratory systems.

Option 3: Prawn - Prawns are crustaceans and have a more complex body structure compared to roundworms. They have a well-developed segmented body, specialized appendages, and complex digestive, respiratory, and circulatory systems.

Option 4: Snail - Snails are mollusks and, like prawns, have a more complex body structure compared to roundworms. They have a co