LARVAE OF ARBACIA PUNCTULATA 331 



above, and a-a" to the last two. In whatever position the 

 pluteus Hes some foreshortening must necessarily occur. Ver- 

 non ('95) selected measurements taken from the side as most 

 nearly accurate; but on account of a divergence laterally of the 

 arms at the oral end of the pluteus, some error must neces- 

 sarily occur under this mode of treatment. Moreover, by this 

 method, measurements could be taken only on one side of the 

 animal. Since one of the purposes of the present investigation 

 was to determine any 'irregularities in the skeleton of each indi- 

 vidual, measurements from both sides of the body were desir- 

 able. By selecting only those specimens which lay in one definite 

 position, — that is, flat against the slide with anal surface directed 

 upwards, the slight error due to foreshortening was reduced to a 

 minimum. 



Figs, a and b 

 EXPERIMENTAL 



The summer egg-production of Arbacia at Woods Hole during 

 1915 was divided into two periods. The periods of sexual ma- 

 turity were of longer duration for the males than for the females, 

 as the former developed somewhat in advance of the latter and 

 remained active later. In 1914, an early period began in the 

 latter part of May while the following summer the first mature 

 females were not obtained until about the sixth of June. In 

 1915, about a week after the first ripe specimens were obtained, 

 practically all were mature. During the next two weeks nearly 

 every female opened possessed ovaries filled with ripe eggs and 

 the males extruded exceedingly active sperm.- A gradual de- 

 cline in the percentage of mature individuals then occurred until 

 the middle of July when scarcely one could be obtained. 



