Evolution of body covering and locomotor organs

Body covering protects, and also takes part in breathing, sensation, heat regulation and excretion. Vertebrate skin consists of the epidermis (from ectoderm) and the dermis (from mesoderm). In fish the skin has mucous glands and the body is covered with scales: placoid scales in cartilaginous fish, bony scales in bony fish. Amphibian skin is thin, moist and takes part in breathing; reptile skin is dry, covered with horny scales and does not breathe; birds have feathers (an epidermal product) and the preen gland; mammals have hair and sebaceous, sweat and mammary glands. Support and movement: unicellular organisms have needle-like internal skeletons and flagella, annelids a hydrostatic skeleton and a skin-muscle sac, arthropods a chitinous exoskeleton, chordates an internal skeleton – the notochord, then the vertebral column. From fish to mammals, fins became legs, wings or arms; birds have a keel and light hollow bones adapted to flight. Mammals usually have seven neck vertebrae (in the giraffe as well as in the mouse).