The Relations between Marine Animai and Vegetable Life. 



407 



on the arm leugth. It was also shown that, in contradistinctiou to 

 the body lengths, the arm lengths vvere greatly affected by the number 

 of larvse developing together in a given volume of water. In order 

 to get rid of this very variable environmental factor, it was there- 

 fore neeessary to introduce a correetion, whereby every arm length, 

 whatever the number of larvae which had been growing together, 

 was converted into its theoretical value at infinite larvai dilution. 

 The same correetion has been introdueed in the aual arm length 

 values obtained in the present instance, and these corrected values 

 are given in the last column of the table at the end of the paper. 

 In the last column but one are given the mean uncorrected arm 

 length values. It is to be noted that the aetual length of the arm 

 was always calculated as a percentage on the body length of the 

 larva, and hence these mean numbers represent the percentage, and 

 not the absolute arm lengths. 



It will be convenient to briefly discuss the arm length data in 

 the same order as was previously observed with those of the body 

 length. In the following table are given the variations produced by 

 the action of algae and diatoms upon the water. These values are 

 calculated as percentages on the (corrected) arm lengths of the larvse 

 grown under normal conditions. 



Numbers of Expts. 



Treatment of water 



"/o Variations in arra 

 length 



Mean 

 Varia- 

 tion in 



arm 

 lengtli 



40, 39, 9, 9;ì 



43,41, 108,24, 89, 2 



94, 95, 9G, 132 



54, 90, 92, 91 



42, 68, 133 



55, 63, 65, 99, 100, 

 123, 124, 125, 126 



73, 129 



Water previously exposed to action +3.9, +24.7, +24.7, 



of Ulva 



Ulva present during larvai develop- 

 ment 



Water previously exposed to action 

 of Gelidium ete. 



Gelidium etc. present during de- 

 velopment 



Gelidium + Ulva present during 

 development 



Water filtered through sand con- 

 taining vegetable growth 



Water previously kept some weeks 

 in diffuse liglit 



— 23.8 



—.3, —.5, —.6, +66.7, 

 — 21.8,-3.9 



— 27.0, —19.1, +0.0, 

 + 12.3 



+ 6.4, —9.9, +8.1, 

 — 5.1 



+ 18..5, —12.2, +12.6 



— 8.5,— 6.5, +.1,-21.4, 



— 25.4, —2.4, —7.7 



+ 24.1, +6.4 



+ 20.6, —11.3 



27* 



+ 7.4 

 + 6.6 



— 8.4 



— .1 

 + 6.3 



— 4.6 



+ 4.6 



