MOLLUSCA. 313 



ternal cartilage or fulcrum. This structure is white and 

 semi-pellucid, and consists of a broad base,, surmounted 

 by a crest or sail. The base is oval, and marked with 

 numerous and very regular concentric striae; it is 

 thick, and contains between its layers, some fluid, and 

 a quantity of air. The crest, a thin transparent carti- 

 lage of an arched form, stands erect, and passes ob- 

 liquely across the base, in a longitudinal direction it 

 is admirably adapted (or, to use a nautical term, " braced 

 up") to catch the wind from whatever quarter it may 

 blow, its position being not unlike that of a lateen 

 sail. 



The entire fulcrum, or skeleton, is invested in soft 

 parts of a purple-green colour. The upper surface of 

 the base is covered with a smooth membrane, which also 

 overlaps the margin, but in a denser and more floating 

 form, and is continued over the entire surface of the 

 crest; the centre of its inferior surface, which is 

 concave, is occupied by a brown mass of viscera, covered 

 with a thick mat of colourless tubular papilla ; whilst 

 its circumference is furnished with slender blue ten- 

 tacles, or cilia, which, though short, are sufficiently 

 long to project beyond the loose marginal membrane. 

 A narrow alimentary canal, placed in the midst of the 

 papillous appendages, extends the entire length of the 

 body, and has several lateral tributary ducts. It has 

 an orifice in its centre, which in some specimens of 

 this species I have observed to be merely a small slit, 

 while in others it projects in the form of a membranous 

 tube. 



When on the ocean, the Velella floats passively upon 

 the broad base of its fulcrum, with its crest raised above 

 the surface. It gives no sign of animation when 

 captured ; nor has it a stinging, or any luminous 



