296 ZOOLOGY. 



that of a " sail." Should this inflated membrane be 

 removed or materially injured, the shell sinks. In its 

 natural element, the shell rests upon its convex surface, 

 or keel, and is immersed; the aeriferous float being 

 alone visible above the surface. 



The contained animal (a trachelipod) has a muscular 

 attachment to its shell only by a small portion of its 

 back. There is a short bifurcated tentacle on each side 

 the head ; and the mouth is a vertical fissure, its mar- 

 gins beset with a row of rigid hairs which are some- 

 times inverted and concealed, and at others projected 

 forwards, the opposite rows crossing or interlacing one 

 with the other. A broad membrane (a foot or branchia) 

 arises from the under part of the neck, and to this the 

 float is attached by a slender base. The stomach is a 

 very distinct and perfect organ ; in some examples we 

 examined, its contents were minute brown shells. 



The body of this mollusc contains a very blue liquid, 

 which, when the animal is punctured, exudes to the 

 amount of three or four large drops. It is readily 

 diffused through water or colourless spirit to the 

 former it communicates a faint tinge of its own peculiar 

 hue, and to the latter a pink colour with a purple shade. 

 It communicates its colour to paper, and may be con- 

 veniently used as a blue ink : several memoranda and 

 pages of my journal, written with this fluid, have, after 

 a lapse of more than five years, retained their original 

 appearance, both in colour and intenseness. For this 

 use, however, it must be employed from the recent 

 animal, as it will not keep in any quantity, but becomes 

 thin and discoloured. It is believed that this fluid is 

 analogous, in use, to the black secretion which the 

 cuttle-fish pours forth to obscure the water and elude 

 the pursuit of its enemies ; but this opinion must be 



