From an evolutionary point of view, the term "primitive" refers to characteristics or traits that are similar to those found in early ancestors. In this context, the most primitive animal among the given options is the leech (option 1).
Leeches belong to the group of animals known as annelids, which are segmented worms. Annelids are considered primitive because they possess basic features and structures that were present in early forms of life. These include a simple body plan, lack of specialized organs, and a relatively primitive nervous system. Additionally, leeches have a simplified circulatory system and reproduce through external fertilization.
On the other hand, the other options (octopus, snail, and spider) represent more advanced groups of animals. Octopuses are mollusks and have complex nervous systems and highly developed eyes. Snails are also mollusks and possess a coiled shell and a more advanced digestive system compared to leeches. Spiders, on the other hand, belong to the arachnid group, which has specialized structures like fangs and silk-producing glands.
Therefore, based on the provided options, the leech is considered the most primitive animal from an evolutionary standpoint.