In insects and some other terrestrial arthropods, a small opening through which air is taken into the tracheae. Insects have several spiracles, arranged along the sides of the abdomen.
Crystalline or mineral deposits found in sponges, sea cucumbers, or urochordates. They are structural components in many sponges, and may serve a protective function in other organisms.
Support structure in animals, against which the force of muscles acts. Vertebrates have a skeleton of bone or cartilage; arthropods have one made of chitin; while many other invertebrates use a ...
Opening in molluscs or in urochordates which draws water into the body cavity. In many molluscs, the siphon may be used to expel water forcibly, providing a means of propulsion.
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