294 Royal Society :— 
organ of Bojanus. It forms neither a sac nor a reservoir, as it has 
been stated to do ; and these phrases, as well as ‘ purpuriferous vein,’ 
should be rejected, because the organ is simply extended over the 
surface. 
Large elongated cells, placed perpendicularly side by side on the 
surface of the pallial cavity in the direction of its greatest diameter, 
compose its tissue. They form about two or three layers, the most 
exterior of which, covered with vibratile cilia, presents the most 
developed cells. Below lies a very rich capillary network, which 
distributes the blood coming from the organ of Bojanus and the 
neighbouring parts of the mantle to the. branchize. The cells, when 
they have reached maturity, fall into the pallial cavity, become 
endosmotically distended, burst, and mingle their contents with the 
other mucus which already existed there. This independent and 
isolated shedding of the histological elements constitutes the secre- 
tion of the dye-stuff, which, it is obvious, is not produced by a com- 
pound gland, or indeed by any gland in the proper sense of the 
word, but by a glandular portion of the pallial surface. It is the 
granular but soluble matter contained in these cells which possesses 
singular properties, and constitutes the dye-stuff. 
The peculiar layer whose position has just been indicated is not 
special, anatomically speaking, to the two genera Murex and Pur- 
pura; and this is important if, in looking at the matter morpholo- 
gically, a similar part of the surface of the mantle of most gaste- 
ropods appears to produce a substance of like histological character, 
but different in its properties. In the Aplysize and the Snails it is 
naturally coloured, whilst in Turbo littoralis and Trochus cinereus 
it is colourless, and undergoes no modification by the action of the 
solar rays. 
Thus, then, it is incorrect to say, with some chemists, that, ana- 
tomically speaking, the purple dye-stuff is yielded by the kidneys of 
Mollusca. 
Anatomical investigation has led to the recognition in the genera 
Murex and Purpura of a peculiar anal gland placed alongside the 
rectum, and opening by a terminal pore close to the anus. This 
gland, which does not seem to have been described hitherto, is in 
structure and the arborescent disposition of its secretory ceeca, a 
well-defined gland ; and by this very circumstance it is impossible to 
confound it with the purpuriferous organ. 
Properties of the Purple Dye-stuff—A very curious fact, known 
from all antiquity, since the very existence of the dye depends upon 
it, is the transformation of the dye-stuff by the action of the solar 
rays. In the living animal this substance is at first colourless, or 
more or less yellowish; exposed to the light of the sun, in a moist 
state it acquires a pure violet hue; in a word, it is photogenic. 
The solar action causes the three simple colours to be developed 
successively, and in the following order, yellow, blue, and red. 
Between these, the compound colours green and violet which result 
from their mixture, are obtained with the greatest distinctness if the 
action is slow. But whilst the yellow disappears by prolonged 
