The Relations between Marine Animai and Vegetable Life. 349 



does not at ali times aud under ali conditions afford a just criterion 

 as to the effects of a particulav water, wliether favourable or otlier- 

 wise, upon the growth of animai lite in general. Thus in the third 

 eoluniu of the above table are given the percentage numbers, on the 

 ova used, of the larvfe which survived to eight days' growth. It will 

 be seen that in ali but the last exi)eriment in the table, in which 

 the amount of Ulva present was excessive, 69^ and upwards of the 

 ova developed into full grown larvje. Now as a mean of the sixteen 

 experiments made from tinie to time during the course of six months 

 with larva' growing in normal tank water under average conditions, 

 it was found that only 66.6 «i; of the ova used developed into fuU^ 

 grown larvse. As in the present series, excluding the last experiment, 

 it is found that on an average 86^ of the ova so develop, it follows 

 that the addition of moderate amounts of Ulva has caused about 

 20^ more of the ova to develop to maturity. 



One or two other observations hearing on the effects of Ulva 

 may bere be cited. Thus in Exp. 67 a mollusc, CeritJiium vulgatum^ 

 weighing 6.2 gni, and with about Ssq.cni. of Ulva growing from its 

 shell, was placed in two litres of water containing fertilised ova. 

 The amount of Ulva was insufficient to absorb ali the ammonia 

 excreted by the mollusc, as after eight days the amounts of free and 

 organic ammonia present were respectively 1.295 and .181 mgm. per 

 litre, as compared with the .528 and .296 mgm. present in the case 

 of larvie grown under normal conditions. Nevertheless the larvai 

 grown in company with the mollusc were ouly .^% smaller than the 

 normal. Again, in Exp. 109, a similar mollusc, weighing 5.6 gm., and 

 with 15sq.cm. of Ulva growing from its shell, was placed with the 

 fertilised ova in 21/2 Htres of water. The resulting larvse were 3.6^ 

 smaller than the normal. In Exp. 110, however, in which the ova 

 were allowed to develop with another mollusc of the same species, 

 weighing 6.5 gm., but without any Ulva growing from its shell, the 

 resulting larva3 were 12.6^ smaller than the normal. The presence 

 of the Ulva seems therefore to bave made a differeuce of ^% in the 

 size of the larvse. 



b. The Effects of Other Kinds of Algse. 



As has already been mentioned, Gosse was able to keep other 

 algse, such as certain red weeds, alive in bis aquaria, and he at 

 first thought them to be more favourable than the green weeds. 

 Warington also succeeded perfectly in growing red weeds in his 



