ZooL.— Vol. III.] BANCROFT— COMPOUND ASCIDIANS. 143 



enemies and covering them. The edges were the most 

 vigorous parts of the colonies. Thus it was frequently 

 seen that the edges of one colony would grow onto new 

 territory, while the less vigorous central parts would suc- 

 cumb to the attacks of enemies and die. Occasionally the 

 central parts would lose vigor and die of their own accord. 



Length of Life of Botryllus Colonies. 



The maximum age of Botryllus colonies seems to be less 

 than a year. The oldest ones of which the writer has 

 records were six months and twenty-three days old. Some 

 of these were in fair condition when observed for the last 

 time, and undoubtedly lived considerably longer. The 

 great majority of the colonies, however, died before they 

 were so old, in spite of all the care that could be bestowed 

 on them, such as removing competing organisms from the 

 slides and giving them a perfectly natural environment. 

 The cause of death was apparently simply old age. They 

 had lost their vigor, and for some time before death had 

 been decreasing in size, until finally they could no longer 

 assimilate enough food to keep them healthy; and so died. 

 At death, the colonies were never very large, but often had 

 a fair size, consisting of from six to ten systems. No such 

 thing as an aestivation or hibernation of the colonies, during 

 which they might lie dormant for a period, and then grow 

 with renewed vigor, was observed; although during the 

 same period such phenomena were observed in Botrylloides. 

 Accordingly, all things considered, it seems certain that, 

 in the great majority of cases, the age of a Botryllus colony 

 is under a year. 



in. Color Variation. 



Most of the statements recorded in this and the following 

 sections are based upon the observation of three families, 

 each composed of a single mother colony, and the brother 

 and sister colonies derived from the larvse extruded by a 



