234 Dr. A. Holt. Colouring Matters in the 



It will be observed, that while the pigment from Diazona in some respects 

 agrees with the 66' body, in other respects it more resembles the 77' isomer, 

 which gives blue solutions in solvents, the colour being not unlike that of 

 indigo. Possibly the Diazona pigment is some other isomer, or an indigo 

 with a different number of substituted hydrogen atoms, but it is impossible 

 to decide this point without far larger supplies of material. 



For the sake of comparison, in the figure (p. 229), the positions of the 

 absorption bands for the violet or blue solutions obtained from Diazona, 

 Bonellia, and Purpura are shown. 



2. Origin and Formation of the Violet Pigment. 



The experimental evidence so far available does not enable one to ascribe 

 any certain origin to the violet pigment nor to account for its development 

 in such different organisms as Mollusca (Murex and Purpura), Vermes 

 (Bonellia), and Tunicata (Diazona). Nevertheless it may be useful to collect 

 such evidence as there is at present to hand. 



In the case of Murex brandaris it seems to be well established that the 

 colour has a photogenetic origin, but in Murex irunculus this is not the case, 

 according to Negri (loc. cit.). In Purpura lajpillus the pigment is produced 

 both by the action of sunlight, and by hydrochloric acid in the dark, this latter 

 observation agreeing with the behaviour of Bonellia viridis according to Sorby. 

 Further, the pigments produced photogenetically in these organisms are 

 uniformly insoluble in alcohol, while those resulting from the action of acids 

 are soluble. In the case of Diazona it is by no means certain that the pig- 

 ment has a photogenetic origin. Prof. Plerdman has recorded the gradual 

 production of violet colour in the living organism under the influence of sun- 

 light, the original yellow-green tint changing first to blue green, then indigo 

 blue, and finally a dull or dirty violet, but he is of opinion that this colour- 

 change attends a moribund condition. There is, however, no evidence that 

 this change would not have taken place in the dark. Natural violet-coloured 

 specimens of the Ascidian have been obtained alive in the Mediterranean, 

 near Naples, and also grey-green specimens which have remained unaltered 

 after preservation in alcohol* but these natural violet specimens do not 

 appear to be healthy, and hence the formation of the dyestuff may 

 accompany or result from a metabolic change. In alcohol the colour is 

 produced in the dark, for the specimens were placed in a closed tank 

 immediately after they were collected. Microscopic examination of an 

 alcohol-preserved specimen shows the purple colouring matter apparently 

 precipitated in the spherical pigment cells of the test, these cells in the inner 

 * See Herdman, ' Linn. Soc. Joum.,' 1913. 



