W. G. Van Name — Bermuda Ascidians. 375 



It forms flat, incrusting colonies, 2 or 2.5 mm in thickness, and 

 sometimes 7o mm across. There is great variation in respect to the 

 closeness with which the zooids are arranged in the colony in differ- 

 ent specimens. Where they are placed close together the zooids are 

 nearly cylindrical and have a nearly upright position ; where they are 

 less crowded, they lie more obliquely, with the anterior end turned 

 up, so that the body is curved. There is also great variation in the 

 number and conspicuousness of the test-vessels and their bulbs, and 

 in some colonies the young zooids, or buds, appear between the rows 

 of adults arranged with nearly as great regularity as the adults 

 themselves, but often quite differently colored. 



In addition to these variations the color varieties are almost 

 innumerable. Not much weight can be placed on such differences 

 in the case of this family. They do not constitute true varieties, 

 but are mere individual peculiarities. The work of Pizon (14) on 

 certain species of Botryllus emphasizes this fact and shows that the 

 colors of the same individual may change from time to time. 



In the majority of specimens the zooids are colored some shade of 

 purple, purplish brown, or purplish red ; sometimes so dark as to be 

 almost black ; at other times very pale and light colored. When 

 the zooids are deeply tinted the test is usually dark colored and 

 pervaded with more or less of the same tint which predominates in 

 the zooids, so that it loses a good deal of its transparency. In light 

 colored specimens it is often nearly colorless, allowing every detail 

 of the external anatomy of the zooids and the test vessels to be seen 

 with great clearness. 



The purple pigment which gives the zooids their color is in part 

 diffused through the tissues, but is chiefly contained in cells which 

 occur most abundantly in the mantle near the anterior ends of the 

 zooids. They are also present in other parts of the zooids, especially 

 along the transverse vessels of the branchial sac, and on the walls of 

 the end bulbs of the test-vessels. The purple pigment is also con- 

 tained in many of the blood corpuscles, and in the cells contained 

 in the bulbs of the test-vessels. The extent of its distribution is very 

 variable. 



In some cases this purple pigment is replaced by a light bluish 

 grey pigment, but in preserved specimens this changes to purple. 



In addition to this ground color the zooids are usually, though not 

 always, marked with a light colored pigment. It is usually a pecu- 

 culiar and very pure white, which is contained in opaque oval cells 

 of the same size as those containing the ground color. They either 



