LARVAE OF ARBACIA PUNCTULATA 421 



larvae aged 120 hours, due to the fact that some have reached 

 their maximum and resorption of skeleton has commenced. 

 The lowest point attained at this time by any control still un- 

 dergoing increase of s'ze is 67.55, Experiment 19, Plot 1, FF. 

 The wide variation exhibited by the control cultures may be 

 ascribed partly to the fact that although they were raised under 

 conditions as nearly standard as possible, some fluctuations in 

 the environment could not be avoided. 



No attempt was made during the course of the experiments 

 to regulate the density of the sea-water, — a factor which the 

 experiments on dilution and concentration show has a profound 

 influence upon growth and development, and, under the con- 

 ditions usually existing in the Woods Hole region, may produce 

 a variation of about five per cent. 



Fluctuations in the temperature were reduced to a minimum 

 by keeping the finger-bowls in which the sea-urchins were 

 raised surrounded by running sea-water. But this did not 

 eliminate all variations in temperature since a gradual rise 

 occurred during the course of the summer. On June 12, the 

 date of the first determination for 1915, th^ sea- water regis- 

 tered 16.5°C. On June 20 it was 17.5°C., on June 27, 18.5°, 

 on July 5, 19.5°, on July 16, 20.5° and on July 22, 21.5°. The 

 highest point, 21.6°, was recorded on July 23. During the 

 following week the temperature fell somewhat, and on July 28, 

 the date of the last observation, was 20.8°. 



Unfortunately, owing to lack of time, no experiments dealing 

 with temperature as a separate factor could be carried out; 

 hence no estimation can be made of the range of variation pro- 

 duced by these changes; but it was observed that the individuals 

 in the cultures kept surrounded by running sea-water were 

 more \dgorous than were those exposed to the fluctuations 

 ordinarily occurring in the laboratory. 



Moreover, during the summers of 1914 and 1915, there was 

 a ^\ide difference in the size and vigor of the larvae raised at 

 different seasons, confirming the conclusions of previous in- 

 vestigators that there is a seasonal variation. During the 

 early period they grew rapidly and reached a high maximum 



THE JOURNAL OF MORPHOLOGY, VOL. 30, NO. 1 



