370 THE BIOLOGY OF MARINE ANIMALS 



^/-tubocurarine. It is suggested that rhythmic ciliary movement is controlled 

 by acetylcholine (24, 84, 1 15). 



Effect of External Factors 



Ciliary activity is affected by various environmental factors, notably 

 ionic concentrations, temperature variations and oxygen supply. 



Cilia are very sensitive to changes in the hydrogen ion concentration 

 of the medium, an increase in acidity bringing the cilia to rest. The effective 

 concentration for stopping ciliary beat varies with the tissue and is related 

 to that of the medium which normally bathes the cilia {vide p. 62). The 

 ciliary arrest which occurs in lamellibranch gills when the animals are out 

 of water for some time results from accumulation of C0 2 ; this has as its 

 consequence a reduction in oxygen consumption. Marine animals are not 

 normally subject to variations in the relative concentrations of cations 

 other than H + unless they live in estuarine environments. An increase in 

 the concentration of potassium accelerates the beat; in the absence of 

 calcium ciliary beat declines; changes in magnesium concentrations have 

 dissimilar effects in different tissues. A discussion of this problem is given 

 by Gray (51). 



Raising the temperature between 0-34 c C increases the rate of ciliary 

 beat. Above 37-5 c C the velocity falls off and heat rigor sets in. The Q l0 

 for the velocity of the frontal cilia of Mytilus varies from 3T in the tempera- 

 ture range 0-10 C to 1-95 in the range 20-30 c C. As the temperature is 

 altered the rate of oxygen consumption varies directly as the velocity of 

 ciliary movement. Ciliary movement persists for about 30 min in the 

 absence of oxygen, but oxygen is necessary for prolonged activity. 



Like other contractile processes, ciliary activity is sensitive to gross 

 changes in hydrostatic pressure (cf. p. 23). When the pressure is suddenly 

 raised by 70 atm or more, rate of ciliary beat is immediately increased, and 

 returns more slowly to resting level (lateral cilia of Mytilus gill). Subsequent 

 decompression produces a reduction of frequency below normal, followed 

 by slow recovery. Much greater pressures (> 300 atm) have a deleterious 

 effect and permanent injury sets in (96). 



Ciliary Locomotion 



It is only in small aquatic animals that cilia are effective for locomotion. 

 The amount of work done against gravity in sea water is relatively small, 

 since the specific gravity of marine animals is generally not much higher 

 than that of the medium {see Table 9.4, p. 404, for some representative 

 values). But, on the other hand, a considerable amount of energy must be 

 expended in overcoming the viscous resistance of the water. 



When a ciliated organism is free to move it will tend to progress in a 

 direction opposite to that of the effective beat of the cilia, whereas the 

 water will tend to go in an opposite direction. For a given species of animal 

 the resultant initial velocity will be directly proportional to its mass. In 

 very small organisms having a low density, full speed will be attained 



