The occurrence of cilia and flagella 2. The dimensions of some cilia and flagella 14 3. The dimensions of the component parts of some cilia and flagella 4. The dimensions of some centrioles and basal bodies 5. Temperature and the activity of Stentor membranelles 6. Viscosity and the activity of Stentor membranelles 7. Ciliary response in Opalina to direct electric current 8. The minimum pH at which cilia of Mya will function and the normal pH of their surroundings 9. Some hormone substances and ciliary activities 111 10. Veratine and the activity of lateral cilia of Mytilus gills 11. Alkaloids, glycosides and other chemicals and ciliary activity 12. The rate of propagation of the contraction wave in some cilia and flagella 160 13. The rate of transmission of metachronal waves 181 14. Parameters of ciliary activity in parts of the membranelle row oi Stentor 186 Xlll CHAPTER 1 INTRODUCTION Cilia seem to have been seen for the first time by the Dutch microscopist Leeuwenhoek in 1675. In a letter to the Royal Society (Leeuwenhoek, 1677), he described the incredibly thin feet or little legs by means of which a small animalcule, which Dobell (1932) believes to have been a ciliated protozoon, moved through the water. Cilia, or the immediate result of their movements, were seen in metazoa at about the same time by de Heide (1684), who described a " motus tremulus " of the gill surface of Mytilus. These authors did not give these little legs any special name, and O. F. Miiller (1786) seems to have been the first to use the name cilia, probably from the similar appearance of a group of cilia to eyelashes. The name flagellum seems to have a more recent origin, and is perhaps due to Dujardin (1841), who used the term flagelliform to describe the appearance of cilia on some protozoa. By 1835 cilia had been found in most of the main animal groups, and the first comprehensive reviews about these organelles were written by Purkinje and Valentin (1835) and Sharpey (1835). To the former authors goes the credit for the discovery of cilia in mammals in 1834. Sharpey described cilia observed by himself and others in protozoa, sponges, coelenterates, ctenophores, turbellarians, rotifers, annelids, molluscs, echinoderms, ectoprocts, tunicates and vertebrates. In addition to reviewing the functions of the cilia in these various groups, Sharpey made some interesting comments on the structure and physiology of cilia. Some authors of that time, including Ehrenberg^(1832) and Purkinje and Valentin (1835), seemed to be of the opinion that ciHa were moved by small muscles attached to the bulbous base of the cilia, while Grant (1835) suggested that they might move by the flowing of water into and out of a tubular organelle. Sharpey, however, put sperm of some plants. In sponges the flagellate cells are positioned so that the water currents produced by flagellar beating draw p, present in at least some members of the group ;-, absent; /, system or stage missing in the group; ?, uncertain (for sensory structures, cilia are presumed to be sensory...