their- Secretions, 625 



was procured, from several univalves belonging to the genera 

 ikf^rex and Purpura; and Colonel Montagu furnishes us 

 with a good account of it in the Purpura lapillus : — " The 

 part containing the colouring matter is a slender longitudinal 

 vein, just under the skin on the back, behind the head, ap- 

 pearing whiter than the rest of the animal. The fluid itself 

 is of the colour and consistence of thick cream. As soon as 

 it is exposed to the air, it becomes of a bright yellow, speedily 

 turns to a pale green, and continues to change imperceptibly, 

 until it assumes a bluish cast, and then a purplish red. With- 

 out the influence of the solar rays, it will go through all these 

 changes in the course of two or three hours ; but the process 

 is much accelerated by exposure to the sun. A portion of 

 the fluid, mixed with diluted vitriolic acid, did not at first 

 appear to have been sensibly affected ; but, by more intimately 

 mixing it in the sun, it became of a pale purple, or purplish 

 red, without any of the intermediate changes. Several marks 

 were now made on fine calico, in order to try if it was pos- 

 sible to discharge the colour by such chemical means as were 

 at hand ; and it was found that, after the colour was fixed at 

 its last natural change, nitrous no more than vitriolic acid had 

 any other effect than that of rather brightening it : aqua regia, 

 with or without solution of tin, and marine acid, produced 

 no change ; nor had fixed or volatile alkali any sensible effect. 

 It does not in the least give out its colour to alcohol, like 

 cochineal, and the succus of the animal of Turbo (Scalaria) 

 clathrus ; but it communicates its very disagreeable odour to 

 it most copiously, so that opening the bottle has been more 

 powerful in its effects on the olfactory nerves than the efflu- 

 via of assafcetida, to which it may be compared. All the 

 markings which had been alkalised and acidulated, together 

 with those to which nothing had been applied, became, after 

 washing in soap and water, of a uniform colour, rather 

 brighter than before, and were fixed at a fine unchangeable 

 crimson." (Ibid., p. 106.) 



' 6. Blainville seems to be of opinion that the coloured 

 secretions now noticed are analogous to the urinary secretion 

 of vertebrated animals {Manuel, p. 160.); but, of the correct- 

 ness of this, doubts may be very reasonably entertained. 

 Besides their purple fluid, the Aplysiae occasionally discharge, 

 but in small quantities, a whitish acrid one, secreted by a 

 gland composed of little round hyaline grains, and emitted 

 by a circular hole that opens externally a little behind the 

 aperture of the oviduct. (Cuvier, Mem., vol. ix. p. 24.) The 

 D6ri5 ejects a similar fluid, which, however, comes from the 

 liver, or from a gland so intimately associated with it as not 

 Vol. V. — No. 29. s s 



