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In Systema Naturæ (1735, 1758), Carolus Linnaeus proposed a definition of the Kingdom Animalia: natural objects that grow, live, and sense. In contrast, plants grow and live but do not sense, while minerals grow but do not live or sense. In this definition of the animal kingdom, species are arranged in classes, families, and genera. This division of organisms into animals and plants was almost unchallenged for more than 100 years. In 1866, Ernst Haeckel came up with the first classification of living beings based on Charles Darwin’s ideas on evolution. He separated the Kingdom Animalia from the new Kingdom Protista based on the possession of tissues and organs. Today, the animal kingdom is restricted to multicellular animals, that is, the Metazoa. This chapter provides an overview of metazoans, including their apomorphies, sexual reproduction and life cycle, and genes involved in organising the metazoan body. It also describes some of the metazoan morphological characters, including cilia and flagella, choanocytes, cell junctions and epithelia.

This chapter tackles the evolution of cellular organization. How did intricate subcellular machines, such as ribosomes, flagella and ion pumps come to exist? How do cells transmit their functional organization to their offspring, and how did that evolve? And where did cellular organization come from in the first place? Contrary to the claims of Intelligent Design, there is ample evidence that random variation of genes winnowed by natural selection played a large role. But conventional views on these matters are too restrictive. Cells transmit structural organization by a hierarchy of mechanisms that includes genes, self-organization, the continuity of membranes and other structures, and a role for the cytoskeleton in helping a growing cell to model new structures upon the existing ones. How cells as we know them originated remains to be discovered, a subject for speculation and wonder but not yet for explication.

This chapter explains the genesis of animal and plant cells. The ancestors of animal and plant cells were bacteria that became associated with smaller bacteria (destined to become mitochondria, which became, as it were, the lungs of the cell). These soft-bodied ancestors became carnivores. By gene transfer and an increase in the number of genes (through fusion with their sister cells), they were endowed with pairs of chromosomes concentrated in a nucleus. They became unicellular animals that reproduced sexually. Among these, certain ones fed on diverse plant-like bacteria that ended by being included in the carnivorous cells as chloroplasts. This series of associations gave rise to various lineages of chlorophyll-containing plants. Others of these carnivorous bacteria, which were more voracious, became cannibals and ingested and tamed the prototypical animals and plants, partly domesticating them. These gave rise to other animal and plant lineages. Among all these lineages, certain ones probably co-opted mobile, strip-shaped bacteria that became their means of propulsion (cilia and flagella).

This chapter examines how the physical properties of water influence and explain the great diversity of swimming performance and mechanisms - from the scale of spermatozoa on up to whales. The key parameters of inertia, viscosity and their manifestation in the critically important Reynolds number are explained and placed in the context of a range of swimming mechanisms, including undulatory movement and fin-based, jet-based, flagellar and ciliary propulsion. The air-water interface also presents an intriguing mechanical challenge for the many organisms that move on top of the water’s surface. The chapter concludes with a brief overview of the burgeoning field of biorobotic swimmers.