ZooL— Vol. III.] BANCROFT— COMPOUND ASCIDIANS. 1 55 



clean, and when the colony has recently grown onto it. 

 Later, worms, Crustacea, diatomes, and other organisms 

 establish themselves under the Botrylliis colony, and help 

 to loosen its attachment. If the substratum is covered with 

 mud and minute animals in the first place, a firm attach- 

 ment is never established. It is evident, then, that neither 

 the thickness of the colony nor the firmness of its attach- 

 ment furnish useful diagnostic characters. 



2. Number of Zooms in a System. 



The number of zooids in a system varies greatly in the 

 same colony at the same time, and also varies with the age 

 of the colony. It may be that there is an inherent tendency 

 in some colonies to have a certain mean number of zooids 

 to a system, but this tendency is not evident at first sight. 

 Lengthy observations and a quantitative treatment would 

 be necessary to demonstrate it. The most obvious varia- 

 tion in the size of the system is found in old colonies where 

 the number of zooids is diminishing. Here, though the 

 number of systems in a given area remains the same, the 

 number of zooids comprising a system is reduced to an 

 average of three or four. 



The size of the intervals between systems depends 

 almost entirely upon the rapidity of growth, and the 

 amount of growing room. In vigorous colonies confined 

 in a small space, there is almost no space between the 

 systems, while in colonies that are free to extend in all 

 directions about one-third of the surface is occupied by the 

 systems. 



3. Size of Zooids and Colonies. 



The variations in the size of both the colonies and the 

 adult zooids depends, so far as could be seen, entirely 

 upon the vigor of the growth and the age of the colony. 

 In very young colonies, containing only a few zooids, the 

 latter never grow as large as the smallest adult zooids in 



(3) January 16, 1903. 



