What are the glands that filter waste out of the bloodstream of an arthropod called?

The correct answer is the green glands, which are specialized excretory structures found in some crustaceans, such as crayfish. These glands play a vital role in maintaining water and ion balance by filtering waste products from the bloodstream. They perform a function similar to that of kidneys in mammals, removing nitrogenous wastes and regulating the body's osmotic pressure.

Green glands are located near the eyes and open into the base of the second antennae, allowing them to effectively remove harmful wastes from the circulatory system. This adaptation is particularly important for aquatic arthropods, as it helps them manage the internal balance of salts and water in their bodies.

Other options refer to different types of glands or terms that may not accurately describe the excretory system in arthropods. For example, excretory glands could be a general term, while caecal glands are involved in digestion rather than excretion. Kidney glands do not pertain to arthropods, as they are associated with vertebrates. Therefore, green glands specifically denote the excretory structures in certain arthropods, making them the correct choice for this question.