XXXvi INTRODUCTION. [CH. 



their prey or concealing tliemselves from fishes or other 

 predaceous animals. The inky cloud which the Cuttle 

 ejects is of a glutinous or viscous nature, and does not 

 readily mix with the water. It seems to be better 

 adapted for entangling small fishes than to cover the 

 retreat of the Cuttle, which is extremely rapid in its 

 movements. The Aplysia, or sea-hare, gives out a 

 purple dye which is also glutinous and has an ofi'ensive 

 smell. As this mollusk crawls but slowly, it may use 

 the dye to obfuscate some more active animal which it 

 may have taken a fancy to make a meal of, all the Nudi- 

 branchs being said to be carnivorous. Several kinds 

 of Planorbis (freshwater snails) yield, on being irritated, 

 a quantity of their own purple blood. These are vege- 

 table-eaters ; and we can therefore conceive no other 

 object in this volmitary blood-letting than to elude ob- 

 servation. A few land-snails (e. g. Bulimus obscurus and 

 Pupa secale) in their young state, as well as some small 

 freshwater bivalves belonging to the genus Pisidium, 

 have their shells covered with mud and other extraneous 

 matter ; but it is difficult to say whether this is the re- 

 sult of design or acyident. One might suppose that the 

 sharp eyes of a bird or a frog would easily detect their 

 prey through this feeble disguis.e. Some marine bivalves 

 (as Lyonsia Norvegica and species of Necera) are covered 

 with a coat of sand, which may to some extent answer 

 the purpose of concealment. If the safety of these 

 animals is ensured by such means, how great must be 

 our admiration of that wonderful yet varied plan of con- 

 trivance which makes the humble Mollusk, as well as 

 Man, the object of Divine care ! 



Renewal of parts. — Some Mollusca, which had been 

 accidentally deprived of theu' feet, tentacles, eyes, and 

 even of their entire heads, have been known to repro- 



