Sea-slugs 279 



covered with little conical points. Both the foot and 

 the flaps of the mantle are used for swimming. The 

 general colour is red-brown, paler above, and with 

 blue-grey spots beneath. The shell is a very thin 

 and flexible transparent plate, similar to that of the 

 last-mentioned species. The coloration of the animal 

 makes it a conspicuous object, and no doubt it would 

 suflfer greatly from the attacks of flshes but for the 

 fact that from its entire surface it can pour out a dis- 

 tinctly acid secretion — so acid, indeed, that blue 

 litmus paper is turned strongly red by it. Acid is 

 much objected to by fishes, and the presence of a very 

 small quantity in otherwise desirable fish - food is 

 sufficient to disgust the fish. When at rest this 

 species frequentl}^ folds up the sides of its broad foot 

 over the body, though the foot is not divided into lobes 

 as in some of those already described. Mr. W. Garstang 

 thinks we may find in this habit a clue to the origin 

 of foot-lobes in other species. " In Oscanius [that is 

 Pleurohranchus] the sides of the foot are frequently 

 folded over the body of the animal when at rest, and 

 this habit is still more marked in Haminea. Now the 

 animals are found to li\'e on muddy bottoms, and a 

 broad flexible expanse of foot is obviously advan- 

 tageous for gliding over such surfaces. But the 

 habits of Aiilysia are different. Ajdysia lives upon 

 alg^e, and for creeping over the narrow stems and 

 fronds of seaweeds a wide plantar surface would be 

 not only unnecessary but disadvantageous ; so we 

 find that the median portion of the originally broad 

 foot has become specialised for creeping purposes, 

 while the lateral portions no longer form part of the 

 plantar surface, but arise from the vertical sides of 



