Classifying Animals

Scientists classify organisms into groups according to their structural and functional features. The information they use can be expressed as a dichotomous key, where each branch yields 2 choices, or expressed as a field guide. Animals can be classified as macroscopic or microscopic. The macroscopic can be divided into the vertebrates, which have an internal skeleton, and invertebrates which do not. The vertebrates can be subdivided according to whether they have constant body temperature or not. Those that do include mammals, and birds the aves group. Reptiles have changing body temperature and internal fertilisation, while amphibians and fish both undergo external fertilisation but have different skin types. An internal skeleton is known as an endoskeleton. Arthropods are a group of invertebrates which have an exoskeleton and jointed legs. Invertebrates having no legs, a soft body and no shell are worms, cnidarians - which include the jellyfish, and porifera otherwise known as the sponges. Molluscs include snails and have a soft body and sometimes a shell, Starfish are invertebrates and belong to the echinoderm group. Fish can be placed into 3 general groups - those with a cartilaginous skeleton - eg. Sharks, those with a bony skeleton - eg. Bream, and the jawless fish eg. Lamprey. Mammals can also be be subdivided into 3 groups. The placental mammals have babies which are well developed at birth and attached to their mothers' placenta. Marsupials babies are born very immature, and the females all have a pouch. The echidna and the platypus both lay eggs. They belong to a group called monotremes