232 SELIG HECHT 
According to this scheme, the pharynx, or branchial sac, of 
Ascidia covers the right side almost completely, whereas the 
renal body and the intestine lie on the left side. The atrial 
cavity extends along the dorsal edge and terminates anteriorly 
in the atrial siphon. Quite unusually for this genus, the heart 
lies on the right side along the ventral edge. 
2. Color 
On first sight Ascidia atra appears to be of a dead black color. 
Closer examination shows that it is really a very deep blue. The 
color is located in the test, on the inner surfaces of the siphons, 
and on the outer faces of the oral tentacles; it may even extend 
beyond the tentacles into the anterior part of the branchial 
sac. In sections of the animal, the blue pigment is shown as a 
thin rim marking the outer edge of the test. 
The coloring matter is insoluble in water. Acetone extracts 
of the test are reddish in appearance, and do not resemble the 
opaque purplish blue seen in sections of the test. The extract 
has the properties of an indicator; it is red with acids and green 
with alkalies, the color change occurring near the neutral point 
(Crozier, ’16). 
The blue pigment is contained in spherical granules, which 
are nearly all of the same size: approximately 3 micra in di- 
ameter. They occur in very compact groups of four to six. 
Although each group may represent a cell containing the pig- 
ment granules, it is difficult to make out any cell substance or 
cell boundaries in thin sections of fresh and fixed tissue. There- 
fore, it seems improbable that the granules as they are found in 
the test are within the living substance of a pigment cell. 
The groupings may, however, represent the remains of meta- 
morphosed cells whose cytoplasm has disintegrated. On the 
basis of such an idea, there should be present in the body of 
Ascidia some living cells which would be the precursors of the 
pigment groups; and, moreover, it should be possible to find 
intermediate stages between the two. 
