108 W. J. CROZIER. 



animals late each afternoon. The tunicates were perceptibly 



blacker on the second day, and rather dense black by the seventh 



day." 



The experiment was several times repeated. The individuals 



concerned were 1.2 to 2 cms. long. 



From such results it might appear that in the case of A. atra 

 light has indeed a direct effect in producing dark pigmentation, 

 as it undoubtedly has in certain molluscs (cf. List's observations, 

 1902, on the superficial pigmentation of mussels; these observa- 

 tions I can confirm from experience with the Bermudan littoral 

 Modiolns}. But it is known that a number of tunicates change 

 color under laboratory conditions (Caullery, '95 ; cf. also Holt, 

 '14). Accordingly, I repeated the above experiment, with the 

 difference that the animals taken from under stones were imme- 

 diately placed in darkened collecting buckets, and in the labora- 

 tory maintained in aquaria in a dark room. By the sixth day, 

 these individuals were as dark as the similar ones exposed in the 

 laboratory to illumination. Hence it cannot be decided from 

 such experiments whether or not light has any direct significance 

 for pigment-deposition in the test of A. atra. 



3. A possible explanation was sought in another direction. 

 Every A. atra found on the under side of a stone slab was rela- 

 tively small, none being more than 4 cms. long. Individuals of 

 this length collected from reef rocks, wharves, buoys and floats, 

 moreover, were comparatively much more robust. As an index 

 of better growth, the thickness of the test on the right side of the 

 body was measured in several of these. This thickness, in the 

 normally situated ascidians, amounted to 2.0 mm.; whereas 

 in those living under stones the thickness of the test was but 

 0.8 mm. Since it is fairly certain that the blue-black granular 

 pigment of the A. atra test derives from metamorphosed vana- 

 dium-containing blood cells (Hecht, 'i8c), and is essentially an 

 excretory product (Crozier, '16), as seems probable for some other 

 ascidian pigments also (Pizon, '01), it is not unreasonable to 

 suspect that malnutrition may be in these instances responsible 

 for faulty pigmentation. Fuchs ('14) observed, in laboratory 

 cultures, that the siphons of young Cionse lengthen remarkably in 



