It is a difficult matter for squid to survive without access to a body of water. Only one species is known to do so at length: Oneiroteuthis demiurgis, a symbiont otherwise known as the dreamer squid. When found in nature, it bears little resemblance to its ocean-bound cousins. Its gray tentacles remain tightly curled around its mantle at all times, causing it to appear as little more than a labyrinthine mound of wrinkles. It spends the majority of its lifespan in total stillness, dreaming about a surrounding world that it never sees with its own eyes.
Due to its near-complete lack of mobility, the dreamer squid must engage in a curious sort of mutualism in order to survive. In exchange for a share of another creature’s metabolic energy, it offers them the complete processing power of its own nervous tissue. This addition often amounts to even more gray matter than the host creature possessed to begin with, resulting in significant leaps in cleverness and intellect. In other words, it serves as a naturally-occurring prosthetic brain, and the "dreams" of the sleeping squid are actually the conscious activity of an entirely different organism.
Some creatures have even developed cavities inside their own bodies that exist specifically to house this cunning cephalopod. A noteworthy example is homo sapiens, the most common host, which lends the entirety of its skull’s interior to a single dreamer squid. This guest wears its host’s skeleton like a portable tide pool, fastening itself to the spine beneath it with two lithe, transparent arms. Once this connection is complete (it often takes place before the host is even born), the two beings become entirely codependent. The hominid’s circulatory system begins to feed the squid, and in return, the squid provides its host with the neurological sophistication necessary to survive.
Despite their total reliance on its nervous system, most humans are completely unaware of their inner cephalopod. The realization of its presence tends to be upsetting, and almost always causes dissociative states of consciousness. These feverish episodes seem to only end in one of two ways: either the human host denies the squid’s existence and forgets the whole affair, or the inner squid is awakened, and all conscious activity abruptly ceases.
Knowledge of the existence of the dreamer squid led to similar theories about the evolution of the human heart.
There's more than one way to untangle the human brain.
Some creatures, like ants, need to work together to be able to dream.