298 THE EFFECT OF FOOD 



or the dimension measured in all the observations on 

 larvae hitherto described, was practically unaffected by 

 the " concentration " of the larvae. This apparent con- 

 tradiction is easily accounted for by the fact that the 

 times of development of the body and of the arms of 

 the larvae is not the same. At moderate temperatures, 

 the body attains about 80 per cent, of its full length by 

 the end of the second day, and 90 per cent, by the end 

 of the third. The arms are practically non-existent at 

 the end of the second day, however, and attain only 65 

 per cent, of their full length by the end of the third. 

 As, therefore, the products of metabolism in the water 

 are practically nil during the first day or two, and only 

 gradually accumulate with progress of time, it follows 

 that the growth of the body tissues is unaffected by 

 them, whilst that of the arm tissues is restrained. 



The influence of the excreta of adult Echinoids upon 

 larval growth was then tested. Echinoids of known 

 weight were kept for a known time in a known volume 

 of water, so that, on determining the absolute effect 

 produced on larvae grown in this water, it was possible 

 to calculate the relative effect produced by unit weight 

 of Echinoid kept for unit time in unit volume of water. 

 On growing larvae in water previously fouled by adult 

 Echinoids of their own species, it was found that, as a 

 mean of five observations, they were diminished in rela- 

 tive size by 2.6 per cent., whilst only 41 per cent, of the 

 ova employed reached the larval stage. On growing 

 them in water fouled by Echinoids of other than their 

 own species, the larvae, as a mean of five observations, 

 were diminished by only 1.9 per cent., whilst 54 per 

 cent, of the ova reached the larval stage. That is to 



