160 Relations between Marine Animal and Vegetable Life. 



diminished in size. The increase in size appears to depend roughly 

 on the amount of contamination of the water. Fish and crabs appear 

 to effecb about ten times as much contamination as molluscs and 

 Holothurians. 



On the other hand, if the water be previously fouled by the sea- 

 urchins StronglyocentrotuS) Sphcerechinus, and Echinus, a decrease 

 amounting on an average to 6'9 per cent, is effected ; but with 

 Arbacia and Dorocidaris there is on an average an increase of 3 per 

 cent. There is indeed considerable evidence to show that an organism 

 exerts a special adverse influence on the members of its own species. 

 The fouling products do not seem to be of the nature of ptomaines, 

 as larvae grown in water fouled by dead sea-urchins were in some 

 cases increased in size, though on an average they were slightly 

 diminished. These dead sea-urchins were found to effect about ten 

 times as much contamination of the water as did the various living 

 animals examined. In connection with these results, reference may 

 be made to some previous experiments,* in which it was found that 

 the addition of small quantities of uric acid and urea to the water 

 caused an increase in the size of the larvae, this increase amounting 

 in one instance to 12*2 per cent. Also it was found that the addition 

 of considerable quantities of C0 2 to the water excited a positive 

 rather than a negative effect on the* size of the larvae. 



Larvee grown in water in which another batch of larvaa had already 

 developed were on an average diminished by 6*9 per cent. 



The evidence as to whether a plutens reacts more to its own pro- 

 ducts of metabolism than to those of other species of plutei was too 

 variable to afford a definite conclusion either way. With water fouled 

 by plutei, the proportion of fertilised ova reaching the full larval stage 

 is as a rule considerably less than the normal ; with that fouled by 

 living Echinoids it is slightly less ; with that fouled by various other 

 animals about the same, and with that fouled by dead Echinoids 

 it is greater than the normal. 



Ammonium chloride exerts a very fatal action on larval growth. 

 Thus with water containing 1 part in 25,400 of the salt, the larvae 

 were diminished 19'0 per cent, in size, and only 28 per cent, of the 

 ova reached the larval stage. Potassium nitrite, if kept below about 

 0*3 gram per litre, and potassium nitrate, if kept below 1 gram, have 

 practically no action. 



Larvee grown in sea water which had been aerated by prolonged 

 agitation were on an average increased by 0*65 per cent. The bac- 

 terial quality of the water seemed practically unaffected by the 

 aeration. 



After keeping water several weeks in darkness the nitrites origin- 

 ally present are much diminished, or disappear altogether. After 

 * 'Phil. Trans.,' B (1895), p. 595. 



