 Hagfish possess a protothymus associated with the pharyngeal velar muscles, which is responsible for a variety of immune responses.The thymus is also present in most other vertebrates with similar structure and function as the human thymus.A second thymus in the neck has been reported sometimes to occur in the mouse As in humans, the guinea pig's thymus naturally atrophies as the animal reaches adulthood, but the athymic hairless guinea pig (which arose from a spontaneous laboratory mutation) possesses no thymic tissue whatsoever, and the organ cavity is replaced with cystic spaces.