294 Royal Society i — 



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 branchiae. 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 Aplysi&e 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 caeca, a 

 well-defined gland ; and by this very circumstance it is impossible to 

 confound it with the purpuriferous organ. 



Properties of the Purple Bye-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 



