INTRODUCTION 9 



veliger, as well as rotifers and some ciliated protozoa, use large 

 cilia for locomotion. Good examples of cilia whose primary 

 function is the collection of food are found in the lamellibranch 

 molluscs where large currents of water are passed between the 

 ctenidial filaments by the action of the lateral cilia, while other 

 cilia on the filaments serve to extract the food particles from the 

 water and carry them away (Fig. 3). Very similar arrangements 

 are found in the *' gill " structures of brachiopods, ascidians and 

 cephalochordates, while some annelids, ectoprocts and entoprocts 

 use ciliated tentacles, which look remarkably like ctenidial 

 filaments in transverse sections, for the same function. The typical 

 rotifers and many protozoa carry their feeding cilia on less 

 specialized structures, and yet produce very considerable feeding 

 currents. Very often the currents produced by these cilia must 

 also satisfy the respiratory needs of the animal, but in others cases 

 special respiratory currents are required, e.g. on the parapodia and 

 " gills " of many polycheate worms, especially in tubicolous types 

 where very large cilia may be present. 



It was pointed out by Gray (1928) that the movement of fluids 

 in tubes by cilia will only be reasonably effective if the radius of the 

 tube is not more than 4 or 5 times the length of the cilia. In larger 

 tubes there will be a central region in which the fluid moves much 

 less rapidly than near the walls of the tube, so that in tubes of very 

 large diameter only a relatively thin film of fluid at the tube surface 

 is in effective motion. In the case of nephridial tubules or the 

 male reproductive ducts of many animals, the diameter is small 

 and cilia can effectively move all the fluid in the tube. 



Sensory structures whose organization is based upon a typical 

 ciliary fibril pattern are known from a variety of animal groups and 

 it is likely that many more will be discovered as more receptor 

 structures are studied with the electron microscope. Several of 

 these structures are described on pp. 32-35. In some protozoa 

 cilia have been modified for supporting functions, but their 

 ciliary origin may be inferred from the arrangement of internal 

 fibrils, at least at the base (see p. 36). 



The functions that cilia can perform are taken over by muscles 

 in the majority of animals. Thus, fluids are moved by muscular 

 contraction in circulatory systems of many animals, while the 

 limbs of some crustaceans are used to create feeding currents from 



