392 H. M. Vernon 



that in moderate doses these products actually had a stimulating 

 influence, so as to bring about by reaction increased anabolic or cou- 

 structive tissue changes. The results obtained in this paper seem 

 to Support this theory, but it is possible that tbey and the former 

 results may be due to entirely dififerent causes. Thus we have seen 

 reason to think that bacteria play an important part in the life of 

 these small larvse, and it may be only through the harmful effeet 

 produced on these injurious bacteria that these products of animai 

 excretion are enabled to exert a favourable influence on larvai growth. 

 Thus it may be that the larvje are themselves unfavourably influenced 

 by the products of excretion, but influenced to a greater degree in 

 the opposite direction by the removal of harmful bacteria. However, 

 the few bacteriological observations made with contaminated waters 

 do not seem to bear out this supposition. Thus the specimen of 

 water used in Exp. 112, which was fouled by molluscs, gave 2530 

 colonies per c.c. after 24 hours incubation, whilst the water used in 

 Exp. 111, which was fouled by sea-anemones, gave 2440. The 

 normal unfouled water gave 2000 colonies. In none of these cases 

 could the colonies be counted after 48 hours, owing to the onset of 

 liquefaction. After eight days larvai growth in these waters, the 

 normal water gave 1040 colonies, and that fouled by anemones 1420. 

 The culture of water fouled by molluscs had liquefied. Thus we see 

 that in every case the number of bacteria was slightly greater in 

 the fouled than in the normal water. A similar result, which will be 

 referred to later on, was also obtained with water fouled by sea-urchins. 

 Though this evidence, and most of that obtained by other in- 

 vestigators into the question of river |)ollution, tend to show that 

 waters contaminated by animai excretions contain more bacteria than 

 uncontaminated, yet it is still possible to hold that the fouling of a water 

 may happen to kill off certain bacteria which are particularly harmful 

 to larvai growth, and that therefore carbonic acid, urie acid, urea, and 

 other products of excretion, act only indirectly. It must be admitted, 

 however, that a direct effect on the larvse seems much more probable. 



The Effects of Echinoids. 

 We have seem that water fouled by certain fish, crabs, molluscs 

 and holothurians exerts a favourable influence on larvai growth, but 

 how do these larvse react to members of their own and other species 

 of the sea-urchin family? The results obtained on this point are given 

 in the accompanying table. 



