The Relations between Marine Animai and Vegetable Life. 397 



that this percentage is probably higher than would geneially be found 

 to occur, in that four of these expeiiments happened to be made 

 with ova in such good condition that of those which developed 

 under normal circumstances, no less than 97^ reached the larvai 

 stage. 



That this efifect on larvai growth is a direct one, and is not due 

 to an indirect influence on bacterial growth, seems probable from 

 the fact that the water used in Exp. 137, which was fouled by a 

 Spt^rerJmms, gave respectively 6600 and 10,900 colonies per ce. after 

 24 and 48 hours incubation, whilst that used in Exp. 135 gave 7000 

 colonies after 24 hours growth, and had liquefìed after 48 hours. The 

 normal unfouled water gave respectively 3340 and 7160 colonies, or 

 only about half the number. 



Chemical analysis showed the fouled water to contain about the 

 same excess of ammonia as when living animals were used. When, 

 however, the values are calculated out for unit weight of animai per 

 litre of water per hour, the enormous contaminating influence of dead 

 organic matter, as compared with that of the living, is rendered 

 apparent. Thus on an average of the three analyses, 2.88 mgm, of 

 free, and 2.04 mgm. of organic ammonia were added to the water, 

 whilst in the twelve analyses of water fouled by living animals, the 

 average additions were respectively only .250 and .230 mgm., or 

 about a tenth as much. 



The Effects of Plutei. 



In the paper already referred to, mention is made of two ex- 

 periments in which the larvai were grovvn in water in which another 

 batch of larvse had already developed. A diminution in size was 

 thus efifected, due presumably to the influence of the products of 

 larvai metabolism. It was determined in the present instance to 

 repeat and extend these observations, especially in order to determine 

 if larvai were more afi'ected by their own excretory products than 

 by those of other species, after the manner of their reaction to the 

 excretions of the adult organisms. The experiments made are given 

 in the accompanying table. 



