A VOYAGE TO I3DIA. 141 



and possess a higher interest in consequence of the sin- 

 gular habits of some of the species. They are double 

 sacks, resembling the beroc in general form, but desti- 

 tute of irridescence. 



The outer sack, or mantle, rarely exceeds an inch in 

 length, and is commonly about half as wide. The inner 

 sack is much smaller, and the interval between these 

 forms a cavity for the water which they breathe, and 

 for some of the viscera. Their visible organs are a trans- 

 parent heart, which can only be seen in the strongest 

 light ; a splendid double row of whitish bead-like cavities 

 forming a spiral line near one extremity, and supposed 

 to be either lungs or ovaries; numerous broad, flat, 

 pearly muscles, barely distinguished by their mistiness, 

 and an alimentary canal as fine as horse-hair, with a 

 slight enlargement at one spot, which has been called 

 a stomach. This enlargement resembles both in size 

 and colour a grain of sand. From the base of the ani- 

 mal arises two longer and four or five shorter conical 

 spines of jelly, curved into hooks at the points, by means 

 of which numerous individuals attach themselves to- 

 gether in double rows like the leaflets of a pinnated leaf. 

 Cords of this kind, composed of forty or fifty animals, 

 were often taken, but they separate and reattach them- 

 selves at pleasure. 



To the gregarious habits of this little mollusque we 

 owe a very singular and striking phenomena, which 

 I have never seen noticed by naturalists, although 



