384 The Development oj Ecliinoderm Larvae. [Feb. 28, 



larvae grown in water containing 4,000 per litre are respectively 13'4 

 and 15'9 per cent, shorter than of those in water containing 500 per 

 litre ; in water containing 17,500 per litre they are 25'9 and 23'3 per 

 cent, shorter, and in water containing over 30,000 per litre 53'0 and 

 43 P 2 per cent, shorter. 



Certain products of metabolism exert a favourable influence on the 

 developments of the larvae, and not, as would be naturally expected, 

 a harmful one. Thus larvae allowed to develop in water containing 

 1 in 70,400 of uric acid are 12'2 per cent, larger than those grown in 

 ordinary water. It is only when tbe proportion of uric acid is in- 

 creased to 1 in 28,000 that an unfavourable influence is exerted, the 

 larvae being in this case 2'1 per cent, smaller than the normal. In 

 water containing about 1 in 60,000 of urea the larvae are about 3 per 

 cent, larger. Again, the amount of carbonic acid gas dissolved in the 

 water may be increased by some 18 per cent., so that it is only just 

 insufficient to kill tiff the larvae entirely, and yet no diminution in the 

 growth is observed. On the other hand, some of the products of 

 metabolism may produce a considerable decrease in the size of the 

 larvae, for larvae gro'wn in water in which other larvae had previously 

 developed were 7'0 per cent, smaller than the normal. 



Thus far the effects of environment on the body-length o'rily of the 

 larvae have been mentioned. As the arm-lengths were also measured, 

 the effects upon them oan also be determined. As a rule, the tissues 

 of the arms seemed to be much more sensitive to environmental con- 

 ditions thaln the tissues of the body, and considerable changes were 

 frequently found to occur in them without any obvious cause. The 

 conclusions to be drawn from these measurements are therefore not 

 so unequivocal as frbm those of the body-length. The most impor- 

 tant point established is that one and the same change of environ- 

 mental condition may frequently produce o'pposite effects on the arm 

 and body-lengths. Thus the body-lengths of larvae allowed to 

 develop at temperatures above 22 are about 2 per cent, smaller than 

 of those at about 20, but the aboral and oral arms are respectively 

 10*8 per cent, and 8*5 per cent. Ibnger. Again, the body-length of 

 larvae developed in diluted water is on an average increased by 

 9'1 per cent., but the arm-lengths are decreased by 7" 7 per cent, and 

 10'5 per cent., or, as the arm-lengths are percentages on the body- 

 lengths, they are practically not affected at all. Also the arm- 

 lengths are not affected to the same extent by the same change of 

 environmental condition. Thus the ratio between the arm-lengths 

 is 4'3 per cent, higher at temperatures above 22 than at those 

 below 18. 



As the number of measurements made was so large, it was thought 

 to be of interest to subject them to statistical examination. It was 

 found that with the body-length and oral arm-length measurements 



