408 



H. M. Vernon 



It will be Seen that in tbree out of the four experiments in 

 which the water was previously exposed to the purifying action of 

 tbe alga Uha^ an iucrease in the arm length resulted. In four out 

 of tbe six experiments in which tbe larva? were grown in contact 

 witb the alga, practìcallv no elfect was produced; but in one of the 

 remaining two experiments a very marked increase in tbe arm length 

 was present, and in tbe otber a considerable decrease. These great 

 variations must be set down to some unknown causes. Tbe action 

 of the Ulva itself would seem to be either nil, or slightly positive. 

 The action of the red weeds would seem to be also very slight, 

 though red weeds in company witb green ones perbaps exert a 

 slightly positive effect. It is to be noted, however, that an apparent 

 absence of effect upon tbe arm length really means that this part 

 of the orgauism is afifected by the environment to the same degree 

 as the body lengtb, wbatever that may bappen to be; for, as already 

 mentioned, these àrm lengtbs are percentages on tbe body length s. 



Tbe effect of filtration of water tbrougb sand impreguated witb 

 vegetable growth would seem to be a distinctly adverse one. Thus 

 in six out of tbe nine experiments made a dimiuution resulted; in 

 one there was no change at ali, and in only two was there au in- 

 crease of arm length. Tbe mean dimiuution is only 4.6^, wbilst 

 the mean increase in tbe body length of the larvse in these ex- 

 periments has previously been sbown to be i.2%. It follows, therefore, 

 that the absolute arm lengtbs of tbe larva? were practically uuchanged. 



The observations on the influence of alg« are therefore somewhat 

 variable and indefinite, but in any case they serve to prove that no 

 marked effect is brought about. The observations on the efifects of 

 bacteria are mucb more definite, as tbe following table will show. 



Numbers of 



Expts. 



Treatment of water 



% Variations in arm 

 length 



Mean Varia- 

 tion in arra 

 lensth 



25, -ól, 61,62 



26, 38, 60 



78, 79, 82, 83, 



118, 116, 119, 



128 



77, 98, 121, 85 



15, 57, 102 

 112, 122, 131 



Water previously exposed to sun and 

 air 



Water previously exposed to sun onlj^ 



kept some weeks in 

 darkness 



Water previously fìltered tlirough as- 

 bestos 



Water previously lieated to 100° C 



- 50°— 77° 



+ 47.8,— 35.2, +11.1, 



— 41.7 



— 6.8, —18.4, —5.9 



+ 26.0, +30.4, —.5, 



— 6.7, +20.9, +27.6, 



+ 42.0,-7.7 



+ 20.2, —11.3, +27.0. 



— 13.6 



— 24.6, —49.5, — 1.7 

 — 1.8, — 4,2, — 4.3, + 6.1 



— 4.5 



— 10.4 

 + 16.5 



+ 5.6 



— 25.3 



— 1.0 



