ZooL— Vol. III.] BANCROFT— COMPOUND ASCIDIANS. 151 



For, though occasionally variable, they are the most 

 constant color characters that a colony possesses. Their 

 position does not seem to depend on blood-vessels. The 

 best example of markings of this kind was found in the 

 two parallel white bands extending from cloaca to branchial 

 orifice (figs. 17, 23). 



(a) The Gr'oiind-Color. — This is almost always lighter 

 (yellower or redder) toward the anterior end of the zooid, 

 around the branchial orifice, and blacker towards the cloacal 

 orifice. The extensive white coating towards the cloacal 

 ends of the zooids must not be mistaken for the ground- 

 color. Its pigment cells are located more superficially, 

 and when it is at all thin the ground-color can be seen 

 beneath it (fig. 8). The differences in the ground-color, 

 although extensive, are not usually abrupt, and gradations 

 are encountered. Thus, in the offspring of colony i (figs. 

 1-15) the ground-color of the vigorous adult colonies 

 varied from black (fig. i) through brown (figs. 7, 11) to 

 yellow (fig. 3) and also to bright brick red, although no rep- 

 resentative of this color was drawn in its prime. ^ The colony 

 represented in fig. 14 would probably have assumed a 

 distinct red color later on. It was not quite adult when 

 drawn, and was lost before it became so. 



(b) The Color Markings of Class (b). — These are 

 almost always present at some period during the life of any 

 colony, and their variations are continuous, all gradations 

 being encountered among the members of the same family. 



(c) The Color Markings of Class (c). — These, and 

 these alone, exhibit discontinuous variations. Only three 

 color markings of this kind have been found among the 

 members of the families examined. In all three cases 

 these markings consist in the covering or replacement of a 

 limited area of the normal dark pigment by a light one. 



iThe comparisons made in this section are only between the normal adult zooids 

 of vigorous colonies; so that all the variations discussed in sections i, 2, 3a, and most 

 of those in 3^, are ruled out. Thus, each colony is considered as having only one color- 

 that of its prime. 



