Arthropoda
Arthropods are invertebrate animals that are best characterized by their exoskeleton and segmented bodies. Arthropods are animals like insects, arachnids, and crustaceans.
Aquatic Arthropoda EXCRETORY system
Aquatic arthropods use a series of metabolic reactions to convert nitrogen to ammonia. Ammonia is extremely toxic so it has to be watered down so the organism does not die. This watered down ammonia is then excreted through a permeable membrane, often times the gills. All crustaceans use this system and because of the fact that it uses a lot of water so that the ammonia is watered down is why crustaceans are not successful land animals.
Terrestrial Arthropoda Excretory system
Terrestrial arthropods use a system called the Malpighian Tubule System to remove wastes from their bodies. It is made up of branching tubules which connect to the blood stream. Keep in mind that arthropods do not have blood but a blood equivalent called hemolymph which is a mix of blood, intercellular fluid, and lymph. Uric acid and nitrogenous waste is removed from the hemolymph by the Malpighian Tubule System and sent to the digestive system. From there is is excreted as feces. Most terrestrial arthropods do not have kidneys so this system works out well for them. However, some do so they are capable of secreting urine as well.
Examples
Spiders (Araneae) are one type of arthropod. They use the Malpighian Tubule System and excrete their wastes through an anus. Some older species of spiders even use small kidneys, but this requires much water.
Another example of arthropods are lobsters (Nephropidae). Lobsters excrete their wastes like most aquatic arthropods do. Wastes are secreted through the gills and digestive glands. However, lobsters are also capable of urinating from organs called green glands at the bases of the antennae.
Horseshoe crabs (Limulidae) are another example of arthropods. They use a mix of methods to excrete wastes from both terrestrial and aquatic arthropods. They secrete wastes through the gills while at the same time are able to excrete feces through and anus and excrete urine as well by using kidneys.