ZooL.— Vol. III.I BANCROFT— compound ASCIDIANS. 145 



The color patterns are numerous and varied. They may 

 be developed either by the absence, in certain regions, of 

 the pigment-cells making up the ground-color of the zooids 

 (figs. 23, 26), or by the addition of certain pigment-cells in 

 places (figs. 16-22). Some of them can be seen to be 

 based upon a structural foundation. In all probability they 

 are all merely accidental concomitants of the morphology 

 of the region containing them, and are of no use in the 

 struggle with the environment. This is seen most dis- 

 tinctly in the case of internal organs which do not show 

 from the outside, but which still contain definite arrange- 

 ments of pigment cells. Thus the endostyle always con- 

 tains a large amount of dark-colored pigment, as can be 

 seen in most of the figures where the branchial orifice is 

 opened wide enough. The stomach is marked also with 

 yellow and black pigment. 



Although but little attention has been devoted to the 

 subject, it was seen that some of the lines that go to make 

 up the external color patterns are coincident with super- 

 ficial blood sinuses. This is true of the line in tl^e mid- 

 dorsal region (figs, i, 7, 10, 21, 23) and the light colored 

 circle about the branchial orifice, most distinctly marked 

 where it cuts the median dorsal line (figs, i, 7, 10, 11, 22). 

 This circle is also superimposed upon the peripharyngeal 

 band. There is no doubt that many other instances could be 

 found by devoting special attention to the subject. 



The variations observed in the colors of Botryllus zooids 

 fall under the following heads: — 



1. Variations of the same zooid at different ages. 



2. Variations among the zooids present in the same 

 colony at the same time. 



3. Variations occurring at different times in the same 

 colony. 



(a) Due to age. 



(b) Due to external influences. 



4. Variations among the brother and sister colonies pro- 

 duced by the same mother colony. 



These classes will be taken up in order. 



