The scientific classification of animals began in the late 18th century. At this time, animals were classified almost entirely by external features, mainly because these are easy to observe. But external features can sometimes be misleading. For example, in the past, comparison of physical features led to whales being classified as fish and some snakes being classified as worms. Presently, animals are classified according to a broader range of characteristics, including their internal anatomy, patterns of development, and genetic makeup. These features provide a much more reliable guide to an animal's place in the living world. They also help to show how different species are linked through evolution. Scientists divide the animal kingdom into approximately 30 groups, each called a phylum (plural phyla). AVertebrates and Invertebrates One phylum of animals, the chordates, has been more intensively studied than has any other, because it comprises nearly all the world's largest and most familiar animals as well as humans. This phylum includes mammals, birds, reptiles, amphibians, and fish together with a collection of lesser-known organisms, such as sea squirts and their relatives (see Tunicates). The feature uniting these animals is that at some stage in their lives, all have a flexible supporting rod, called a notochord, running the length of their bodies. In the great majority of chordates, the notochord is replaced by a series of interlocking bones called vertebrae during early development. These bones form the backbone, and they give these animals their name-the vertebrates. Vertebrates total about 40,000 species. Thanks to their highly developed nervous systems and internal skeletons, they have become very successful on land, sea, and air. Yet vertebrates account for only about 2 percent of animal species. The remaining 98 percent, collectively called invertebrates, are far more numerous and diverse and include an immense variety of animals from sponges, worms, and jellyfish to mollusks and insects. The only feature these diverse creatures share in common is the lack of a backbone. Some invertebrate phyla contain relatively few species. An extreme example is the phylum Placozoa, which contains just one species. Measuring less than 0.5 mm (0.02 in) across, this unique animal was first discovered in 1883 in a saltwater aquarium in Austria. Its flat body consists of just two layers of cells, making it the simplest known member of the animal kingdom, although not the smallest. Another minor phylum, the loriciferans, was classified in 1983 with the chance discovery of a tiny organism dredged up in marine gravel. Several other species of loriciferans have since been identified, but little is known about how they live. At the other end of the spectrum, some invertebrate phyla contain immense numbers of species. These major phyla include the annelids (segmented worms), with 12,000 known species; the nematodes (roundworms), also with 12,000 known species; and the mollusks, including bivalves, snails, and octopuses, with at least 100,000 species. The arthropods, with about 1 million known species, include the insects, spiders, and crustaceans. These figures include only species that have been described and named, which are only a portion of those that actually exist. Some biologists estimate that the total number of nematode species may be as high as a quarter of a million, while the total number of arthropods could exceed 10 million. Compared to vertebrates, most invertebrates are animals of modest dimensions. Giant squids, which are the largest invertebrates, can exceed 12 m (40 ft) in length, but the great majority of invertebrate animals are less than 2.5 cm (1 in) long. Their small size enables them to exploit food sources and infiltrate habitats that larger animals cannot use, but it also leaves them exposed to changing environmental conditions. This is not often a problem in the sea, but it can create difficulties on land. Land-dwelling invertebrates have to cope with the constant threat of drying out, and most of them quickly become inactive in low temperatures.
breathing classification diets happened hold home seasonal simplest spans weather dragonballz