CHAPTER 18 



Cilia 



r 



V J ILIA ARE PERMANENT MOTOR differentiations at the cell surface 



m. y\ which are capable of performing work by their rapid and 



^^^^^r ' usually rhythmical movement. They are often divided into two 

 types: (1) flagella, which are relatively larger organelles and which are present 

 usually singly or in small numbers upon cells; and (2) cilia proper, which are 

 relatively much smaller and occur characteristically in large numbers upon 

 each cell. Typical flagella are common among the Mastigophora of the phylum 

 Protozoa, the choanocytes of the Porifera, the gastroderm of many coelenter- 

 ates, the flame end-bulbs of certain rotifers, the solenocytes of annelids, and the 

 sperm cells of most groups throughout the animal kingdom. Cilia proper are 

 characteristic of the ciliated protozoans and are found o\'er more or less of 

 the body surface of coelenterates, Turbellaria, and Nemertea. In all other 

 phyla of animals except the Nematoda and the Arthropoda (excluding the 

 Onychophora), they are found at certain locations in or on the bodv. 



Ciliary activity is restricted to an aqueous medium, and hence is found 

 only on surfaces which are submerged, or at least covered by an aqueous 

 film. By typical ciliary movement two tvpes of observed results may be ob- 

 tained, depending on the inertia of the ciliated surface. If the latter is small, 

 then a movement of the ciliated surface through the medium, or locomotion, 

 results; if, on the other hand, the ciliated surface is large, or the ciliated 

 structure is not free to move, the external medium is caused to move over 

 the ciliated surface. It is therefore obvious that ciliated surfaces can become 

 most effective for rapid locomotion only in small organisms such as pro- 

 tozoa, ciliated larvae, etc. In such forms acceleration to maximum speed is 

 very rapid, and the organisms stop very quickly on cessation of their ciliary 

 activity. 



Larger organisms may be moved only sluggishly by cilia. Acceleration is 

 generally slow, and the animal fails to come rapidly to a halt on cessation 

 of ciliary beat. Furthermore, unless such larger free-swimming organisms as, 

 for example, ctenophores, possess a density very close to that of the sur- 

 rounding medium, cilia are powerless to serve effectively as locomotory or- 

 ganelles. It is interesting, therefore, that the comb-jelly, Plenrohrachia, with 

 a water content of 94.73 per cent, has a density of 1.02741, very close indeed 

 to that of the surrounding sea water.^^- ^^ 



Cilia are locomotor organelles also in many small worms, in numerous 

 rotifers, and even in certain snails, e.g., in Nassa. 



It is perhaps significant that, whereas activity of most ciliated surfaces is 

 continuous throughout the life-time of the animal, cilia whose primary func- 



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