The continuous genetic change in organisms that results from their adaptation, by natural selection to ever changing environments is called evolution. Animals are heterotrophic, eukaryotic and multicellular. Life cycle of animals include a dominant diploid adult that produces eggs or sperms by meiosis. Animals are capable of complex and rapid movements and they develop from embryos like plants.
Diversity is the abundance in the number of species in a given location. Most animal phyla are invertebrates. Extant (living) animals are divided into 35 phyla. Phyla are split according to their adult and embryological forms. First spilt in evolution is Parazoa and Eumetazoa. Parazoa do not possess true organelles and their body parts are primitive. Eumetazoa have well developed tissues and organs. Animals under radiata have radial symmetry and are diploblastic. Animals under bilateria are bilateral in symmetry and are triploblastic. Many animals such as humans are symmetrical. Acoelomates do not have body cavity, while coelomates have it. Pseudocoelomates have a false cavity.
Embryological changes take place during evolution. Gastrulation is the differentiation of animal tissues into germinal layers. Bilaterally symmetrical animals produce three germ layers: Ectoderm, Endoderm and Mesoderm. Ectoderm becomes the outer surface and nervous tissue of animals. Mesoderm becomes muscle, skeletal and connective tissue. Endoderm becomes the lining of the digestive gut. Diploblastic animals (Cnidaria and Ctenophora) have only endoderm and ectoderm. Platyhelminths, Nematodes, Rotifers, Mollusks, Arthropods and Chordates are triploblastic animals with three germinal layers.
All surviving animals today can be traced back to their ancestors to this time period. About 500 million years ago extant animal body plans were identified on fossil records. Hard body parts were easily identified from the records. Animal diversity has been developed due to increased dependency or the predatory/prey relationship, development of jeans to assist embryonic development and adequate environment oxygen to support active animal lifestyle.
Sponges are marine animals, live singly and attach to a substrate. Coelenterates exhibit radial symmetry. There are three major groups of flatworms: Planarians, flukes and tapeworms. Round worms have false body cavity which gives shape to the worms. Mollusks have two circulatory systems, bilateral symmetry, complete digestive tract and internal organs. Segmented worms have nervous, circulatory and excretory systems in each segment. Insects belong to the most successful phyla – Arthropoda. They are segmented, have jointed appendages and have an exoskeleton composed of chitin. Starfishes are slow moving animals and have unique water vascular system. Vertebrates have a backbone or a vertebral column, humans belong to this phylum.