232 



SCIENCE PROGRESS. 



measure as they contain a very sharply defined calcareous 

 skeleton, of the form shown in the figure. 



X 2S0 



In every case the length of the body AB, and the lengths 

 of the oral and aboral arms AD and AC, were measured. 

 In order to diminish the error of observation as much as 

 possible, fifty larvae were measured in each case and a 

 mean taken ; also, as a rule, several sets of observations 

 were made in respect of each environmental condition in- 

 vestigated. This necessitated a very large number of 

 measurements, so that altogether some ten thousand larvae 

 were measured. The larvae were, as a rule, allowed to 

 develop for eight days before measuring, because the arm 

 lengths reach their maximum length in this period. After 

 this they gradually undergo absorption. 



The environmental condition investigated most thor- 

 oughly was that of temperature, because in the first place 

 this is one which undergoes such frequent and considerable 

 variations under natural conditions, and also some interest- 

 ing and unexpected results were obtained with regard to it, 

 which necessitated more extended inquiry. These un- 

 expected results relate to the temperature of the water at 

 the time of impregnation of the ova. It might be reason- 

 ably expected that the temperature would at this moment 

 have no more influence upon the subsequent size of the 

 larvae than it has at any other time ; but such is not the 

 case. So sensitive are the ova and spermatozoa to tem- 



