ZooL— Vol. III.] BANCROFT— COMPOUND ASCIDIANS. 1 73 



characters, there is a series of physiological characters of 

 equal importance. These are the force regulating the size 

 of the systems, and the correlation in growth and repro- 

 duction of the zooids. 



(a) Force Controlling the Systems. 



In every Botrylhis colony the zooids are arranged in 

 systems of from four to twelve zooids around a common 

 cloacal orifice. The manner in which these systems are 

 formed was first clearly described by Krohn (1869, i869«), 

 although Giard (1872), Jourdain (1886), and Pizon (1892- 

 1893, 1899, 1900) have not given him the credit he deserves. 

 From his account, from that of Pizon (1899), as well as 

 from the writer's own observations, it is evident that the 

 systems persist no longer than one generation of zooids. 

 When the new generation first appears, each zooid has a 

 separate cloacal as well as branchial orifice, and the zooids 

 are irregularly arranged. Soon, however, they group 

 themselves into systems, and the cloacal orifices fuse to 

 form the common cloacal opening. 



There is evidently some attractive or repulsive " force," 

 the nature of which is entirely unknown, which in Botryllus 

 causes any number of adjacent zooids up to about twelve 

 to form one system, but which causes a greater number of 

 adjacent zooids to breakup into two or more systems. Now 

 the question arises, is this force of such a nature that it will 

 tend to cause zooids derived from different colonies to help 

 form the same system ? In only one undoubted case^ (fig. 23) 



1 In one case three colonies were dredged, one of which contained zooids of two 

 colors, sometimes present in the same system. It was concluded from the evidence at 

 hand that these colonies had all been derived from the same mother colony, and that 

 the colony with the two kinds of zooids had been derived from the fusion of two sister 

 colonies. As these colonies form the only exception to the general statement made 

 above, that in Botryllus colonies collected by ordinary methods, and, therefore, probably 

 unrelated, do not fuse, the evidence should be given. The three colonies came up in 

 the same haul of the dredge, and repeated dredging in the same locality failed to bring 

 up any more. The writer was immediately struck by the similarity of coloration 

 (although at that time not knowing that sister colonies sometimes have the same color, 

 or that they will fuse). Two of the colonies had exactly the same color and color 

 pattern, and the third was exactly like these two, except that some of its zooids had a 

 light coating of white upon their cloacal ends. A reference to the section on variation 

 will show that it is just this kind of variation which is eminently characteristic of sister 

 colonies. Furthermore, when grafted together or allowed to approach each other by 

 growth, they fused. Hence, in all respects they behaved like sister colonies, and the 

 colony containing the two kinds of zooids was probably derived from two fused sister 

 colonies. 



