APLYSIA. 205 



pedal glands. These differ from the anterior and supra-pedal 

 glands in that each gland possesses a cellular duct into which 

 the large gland cells discharge. The duct is of wide calibre, 

 and its walls consist of large squarish cells bearing long flagella. 

 (Fig. 24, post. ped. gl.) 



Histology and Physiology of the Mantle Glands. 



(a) The Purple Gland. As has already been described 

 under External Characters, a rich purple dye is secreted by 

 unicellular glands situated on the under side of the free edge 

 of the mantle. Secretion takes place only when the animal 

 is irritated. A large quantity of mucus issues with the dye, 

 which mixes slowly with the sea- water and forms a screen under 

 cover of which Aplysia can hide itself or escape. 



The dye is not a permanent colour in the form in which 

 it is given off by the animal. It is therefore not adapted for 

 use as is the dye obtained from Murex, Purpura, etc., though 

 it probably resembles it fairly closely in chemical composition. 

 There is no record in early literature which shows that the 

 ancients either knew of or attempted to use the dye obtained 

 from Aplysia. Their feelings of disgust towards this harmless 

 Mollusc probably prevented the discovery of the dye in those 

 species which produced it.* 



Under the influence of. light the purple changes colour 



and becomes reddish ; acid increases the intensity of colour, 



alkali does not affect it at first, but on subsequent exposure 



to light it becomes rose pink. When a piece of cotton or silk 



material is stained with the dye the colour remains purple for 



a variable period, but ultimately changes, especially if left 



in the light, to a dull brown. The same colour change takes 



place, but much more rapidly, when the purple is mixed with 



alcohol. 



* Aplysia punctata and A. limacina exude purple, but A. depilans only 

 a milk-white fluid. The last named is a common Mediterranean speciec, 

 and is probably the one described in the early literature. 



