194 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



EXTERNAL CHARACTERS. 



(Figs. 1-7.) 



If possible, Aplysia should be examined alive before any 

 attempt is made to dissect it. No one who has seen only the 

 preserved animal can have an adequate idea of the appearance 

 of the living animal, for not only does it lose its colour and the 

 velvety nature of the skin, but it dies in a contracted condition, 

 rarely measuring more than a third of its length when alive. 

 When preserved it assumes the " sitting-hare " position, but 

 it seldom does so during life. Then it resembles a large slug 

 with a face like that of a rhinoceros of grotesque and solemn 

 aspect. The body is relatively broader and the visceral hump 

 higher than in the Slug. 



The colour of the full-grown animal varies, but it usually 

 resembles very closely the particular weed on which the 

 specimen is found. In many it is a rich olive green or brown, 

 with small black spots and irregular greyish flecks. A black 

 spot is due to granules of pigment in the skin, a white or 

 greyish mark to absence of pigment, whereas the green or 

 brown colour is caused by colouring matter dissolved in the 

 cells of the epidermis and underlying layers. Preservative, 

 especially methylated spirit, quickly extracts the soluble 

 pigment, and to a lesser extent also the granular pigment. 

 Hence the grey appearance of preserved specimens. Gautier 

 and Villard* have shown that the soluble colouring matter 

 possesses the same spectrum as chlorophyll, and that it can be 

 traced back to the chlorophyll extracted from the food by the 

 liver cells. If this is so, Aplysia can be said to imitate the weed 

 by making use of the pigments extracted from the weed. 



Not only, however, does Aplysia imitate the colour of the 

 weed, but also its shape. For example, a specimen feeding 



* Gautier and Villard. " Recherches sur le pigment vert jaune du 

 tegument des Aplisies." C. R. Soc. Biol., Paris, T. LVI, p. 1037-1039, 

 1904. 



