398 Wm. s. Marshal] and C. Ï. Vorhies, 



which had not been active for some time), and a comparison of this 

 with the same parts in glands taken from larvae that had, during 

 several days, been rebuilding their cases, thus keeping their spinning 

 glands active. The ease with which the larva of Platyphylax can be 

 induced to repair old or to construct new cases ^) makes it possible to 

 obtain glands which have been active for certain definite periods. 



In working out this part of the study two methods were adopted, 

 one of which, provisional only, was used to ascertain if further work 

 was desirable; after this was determined, these first results were then 

 used, in comparison with the others which we obtained by a second 

 method. In this first method two larvae were taken, and, the case 

 being removed from one of them, they were placed in the same aqua- 

 rium. The exposed larva would begin the construction of a new case 

 which we each day removed, thus forcing it to use its spinning glands 

 continuously for certain definite periods. We were in this way enabled 

 to procure active glands, the length of whose activity we knew, and 

 compare them with the glands from another larva which had not been 

 active, but otherwise under exactly similar conditions. From a study 

 and comparison of such glands it was seen that cytological changes 

 had taken place during both short and long periods of activity and 

 also that these changes were not restricted to either cytoplasm, or 

 nucleus but occured in both. 



'J^his preliminary examination showed us that a more detailed and 

 regular investigation was desirable and for this we adopted a second 

 method. This was to prepare a set of glands, each representing a 

 different period of activity, at intervals of twenty-four hours; maximum 

 length of activity two hundred and forty hours. Another and similar 

 set was prepared and used to compare with the first. In all of these 

 twenty-four hour periods we counted from the time the larva was 

 removed from its case and placed in au aquarium; this was not always 

 accurate, as larvae often remained for two or three hours before be- 

 ginning the construction of their new cases. Finding that after twenty- 

 four hours of activity marked changes had ah^eady taken place, we 



^) For an account of this repair and rebuilding see paper by us in the Biolo- 

 gical Bulletin. 1905. 



