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The kingdom Animalia contains multicellular organisms that are heterotrophic and motile (although some have secondarily adopted a sessile lifestyle). Most animals have bodies differentiated into separate tissues and these animals are known as metazoans or eumetazoans. They have an internal digestive chamber, with one or two openings, the gametes are produced in multicellular sex organs and the zygotes include a blastula stage in their embryonic development. Metazoans do not include the sponges, phylum Porifera, or the Placozoans which have undifferentiated cells.[31] Animal cells do not have a cell wall and do not contain chloroplasts. If a vacuole is present, it is smaller than that found in a plant cell. The body tissues are composed of a great variety of cells such as muscle, nerve and blood cells. Each typically has a cell membrane formed of phospholipids, cytoplasm and a nucleus. Animal tissues can be grouped into four basic types: connective, epithelial, muscle and nervous. Even the simplest invertebrates contain at least two types and body structures and organs of higher animals are formed from multiple tissue types.[32] Connective tissues are fibrous tissues and are made up of cells scattered among non-living material which is called the extracellular matrix. Connective tissue gives shape to organs and holds them in place. The main types are loose connective tissue, adipose tissue, fibrous connective tissue, blood, cartilage and bone. The extracellular matrix contains proteins, one of the commonest of which is collagen, and plays an important part in organizing and maintaining tissues. The matrix can be modified to form a skeleton to support or protect the body. An exoskeleton is a thickened, rigid cuticle which is stiffened by mineralization, as in crustaceans or by the cross-linking of its proteins as in insects. An endoskeleton is internal and is present in all higher animals and many lower ones.[32] Layers of epithelial cells in the epidermis Epithelial tissue is composed of closely packed cells with little intercellular space. They can be squamous (flat), cuboidal or columnar and rest on a basal lamina, a membrane composed of extracellular matrix secreted by the epithelial cells. The cells are bound closely to each other by cell adhesion molecules. One or more layers of epithelial cells line the cavities and surfaces of structures throughout the body including the external surface, the respiratory surface and the gut. The epithelial cells on the external surface of the body typically secrete an extracellular matrix in the form of a cuticle. In simple animals this may just be a coat of glycoproteins.[32] In more advanced animals, many glands are formed of epithelial cells.[33] Muscle cells (myocytes) form the active contractile tissue of the body. Muscle tissue functions to produce force and cause motion, either locomotion or movement within internal organs. Muscle is formed of contractile filaments and is separated into three types; smooth muscle, skeletal muscle and obliquely striated muscle. Smooth muscle has no striations when examined microscopically. It contracts slowly but maintains contractibility over a wide range of stretch lengths. It is found in such organs as sea anemone tentacles and the body wall of sea cucumbers. Cross-striated muscle contracts rapidly but has a limited range of extension. It is found in the movement of appendages and jaws. Obliquely striated muscle is intermediate between the other two. The filaments are staggered and this is the type of muscle found in earthworms that can extend slowly or make rapid contractions.[34] In higher animals striated muscles occur in bundles attached to bone to provide movement and are often arranged in antagonistic sets. Smooth muscle is found in the inner linings of organs and cardiac muscle is found in the heart, allowing it to contract and pump blood round the body. Nervous tissue is composed of many nerve cells known as neurons which transmit information. In some slow-moving radially symmetrical marine animals such as ctenophores and cnidarians (including sea anemones and jellyfish), the nerves form a nerve net, but in most animals they are organized longitudinally into bundles. In simple animals, receptor neurons in the body wall cause a local reaction to a stimulus. In more complex animals, specialised receptor cells such as chemoreceptors and photoreceptors are found in groups and send messages along neurons to other parts of the organism. Neurons can be connected together in ganglia.[35] In higher animals these form the sense organs and there is a central nervous system (brain and spinal cord) and a peripheral nervous system. The latter consists of sensory nerves that transmit information from sense organs and motor nerves that stimulate muscles to contract.[36] |
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