AERATION OF THE SOIL BY EARTHWORMS 395 



Accepting this figure, we only have to weigh worms, divide the 

 result by no and the volume is obtained. This may be very 

 useful when investigating small species of which only a single 

 specimen or at best but few specimens can be obtained. 



The Ratio of the Diameter to the Length of Worms. — 

 This ratio is very difficult to obtain accurately, as worms are 

 capable of considerable extension. In order to determine 

 the length, /, the worms were sprinkled with sand to allow 

 of obtaining a good grip, then gently drawn out until a definite 

 point of resistance was attained. The length was noted. The 

 worms did not appear to suffer in the slightest from this treat- 

 ment. The volumes of the worms were determined by immersing 

 in water in a burette as before and the diameters calculated from 

 the expression 



where d — diameter, /= length, V = volume and it = 3-1416. 

 This assumes that the worm is truly cylindrical, which it is not, 

 since it tapers at the head and tail ; consequently the results are 

 only approximate but the error will be much the same for all 

 the worms. 



Only the larger worms could be treated in this way, as in the 

 case of the small ones any slight error in the determination of 

 the length would produce a proportionately large error in the 

 calculation of the diameter. If the worms could be tested when 

 dead the length could be determined easily enough for all worms, 

 large or small; but if killed in spirit their bodies harden and 

 render the measurement impossible. If killed in other ways, 

 the delicate skin of the worms is apt to give way under the 

 stretching and to yield uncertain results. 



In a series of experiments with ten specimens of A. tonga, the 

 average lengths of which were 20*2 cm. and average diameters 

 0-378 cm., the ratio : 



diameter d C378 1 

 length / ' 2o"2 "" 53" 



In a similar manner ten specimens of O. cyaneum gave the ratio : 



d 0-264 1 

 T i4'9 := 5*" 



Whether or not this close agreement is due to coincidence 

 cannot be definitely decided until further experiments have 



