MOLLUSCA. 79 



for the ravages it commits on ships. These mollusca 

 excavate, in submerged wood, holes or tubes, which it 

 lines with a calcareous matter so hard that it might 

 readily be mistaken for a second shell. Vessels have 

 been sunk by the holes bored through their bottoms, by 

 these animals,' and the only way to guard against their 

 depradations, which they carry on constantly and to 

 such a large extent, is to sheathe the submerged portion 

 of the ship with plates of copper. These mollusca are 

 eatable, and said to be preferrable to the common oyster. 

 The Fire Bodies (pyrosoma elegans), plate 25, fig. 6. 

 This very remarkable family are without shells, and. 

 uniting in great numbers, form a large, hollow cylinder, 

 which, by the alternate contraction and dilatation of the 

 animals that compose it, swims in the water and emits a 

 beautiful iridescent light, seen to great advantage by 

 nighfe. When several of these pyrosomae are swimming 

 together, they appear like an immense luminous globe, 

 arising from the dark bosom of the ocean. The entire 

 length of the animal is seven inches, the circumference 

 one only. When floating quietly, the color is yellow, 

 with a slight mixture of green, but as soon as it contracts 

 in order to move, a fiery glow, resembling the hue of 

 red hot iron is emitted, and which again fades into the 

 changeful shades exhibited by iron as it cools. 



