Comprising about 1,000 cells, it is the most primitive animal to exhibit characteristics that are important in the study of human biology and disease. Though tiny and transparent, C. elegans contains ...

"Affective," in the context of primitive sentient animals, refers to internal "feeling" states that carry positive or negative value, driving behaviors such as approaching beneficial stimuli or ...

This primitive animal has teeth on its tongue, secretes a viscid slime and can tie itself in a knot. It also has four hearts, the nature of which may clarify cardiac function in higher animals ...

Fish appear in the fossil record before four-legged animals (tetrapods), so the ancestor of this group must be at least as primitive as a fish. The most ancient fish were jawless, and since our ...