There are certain organisms which lose the metabolic wastes through their body by diffusion. It includes sponges, coelenterates and echinoderms. There are flame cells which help in excretion and occur in flat worms. The excretory system is unique in round worms which contains canals and tube. Annelids have nephridia as their excretory organ. Arthropods have malphigian tubules as their excretory organ. Prawns have green glands for excretion and vertebrates have kidneys as their excretory organ. The role of excretory organ is to remove unwanted wastes and regulate the water content in body and is known as Osmoregulation. Animals are classified on the basis of type of removal of nitrogenous waste. They can be ammonotelic, ureotelic, uricotelic and aminotelic. Ammonotelic animals are those which excrete ammonia. It includes sponges, ascaris, liver fluke, prawn, bony fish and crocodile etc. Ureotelic animals are those which excrete urea. It includes ascaris, earthworm, mammals including man and turtles etc.Uricotelic animals are those which excrete uric acid. Insects, crustaceans, reptiles and birds are included in it. Aminotelic animals are those which excrete amino acids. Molluscs and echinoderms are included in this group.
Endocrine glands are also known as ductless glands. They secrete hormones which are transported by the blood to the target cells to perform physiological activity. Receptor is an organ which receives changes in the environment and passes the changes to the nervous system. Some of the animals like coelenterates, flat worms, round worms and annelids lack skeleton and have a soft body. Majority of the animals have a skeleton which is of two types that is exo and endo skeleton. Skeleton protects the softer parts of animals.
Exoskeleton is made up of non living material and lies outside the body of animal. It can be made of chitin, calcareous shells or hard plates of arthropods, molluscs and echinoderms respectively. Scales, feather, hair, horns, hoofs and claws of vertebrate are also included in it. Endoskeleton is present in the body of animal. It is composed of connective tissues like the cartilage and bone in vertebrates. In invertebrates they are made up of spicules which may contain calcium or silicon.