Air is briefly stored inside small sacs known as alveoli (sing. They have musk glands and anal sacs. Earthworms have one or two pairs of testes contained within sacs. Mast cells have within them small sacs, or granules, surrounded by membranes. It can also refer to the sperm sacs or testes that contain the semen. Multiple gestation is evaluated by the number of placentae and amniotic sacs present. On inhalation, air travels through the trachea of a bird into the air sacs. During that time, they are dependent on their yolk sacs for nourishment. During this period, the fry survive by consuming the remnants of their yolk sacs. Thus, they are able to use a series of air sacs connected to the lungs. The contracting posterior air sacs can therefore only empty into the dorsobronchi. A few mantispids are parasites of spider egg sacs. The anthers have two or four pollen sacs (locules). A thick coat of muscle fibers that cause rhythmic contractions line the sequestered primitive sacs. The frog immediately suffocated because of the collapse of its lung sacs.