258 MOLLUSCA. 



surface of the sea, boring tubular canals in the direc- 

 tion of the grain of the wood, not for the purpose 

 of feeding on its substance, but merely for the 

 defence of its naked body. The shell is very small, 

 and the mantle protrudes considerably, in the form 

 of a lengthened tube. We have seen a piece of 

 floating timber so perforated, as scarcely to bear 

 being handled ; and the once powerful republic of 

 Holland has repeatedly been threatened with ruin, 

 in the destruction of its dykes by this borer. Ships' 

 bottoms are frequently destroyed in tropical seas 

 from the same cause, and probably the foundering 

 of many a gallant craft, unheard of and unknown, 

 has been the work of the silent and insidious 

 Shipworm. The valves are roughened, and act as 

 an auger. The interior of its cave is always lined 

 with a smooth shelly crust, which is secreted from 

 the mantle, and acts as another shell. Fortunately, 

 both it and the Pholas cannot live in fresh-water, 

 or our bridges would have but a very brief ex- 

 istence. 



To represent the ACEPHALA TUNICATA,* or those 

 which are invested in a leathery coat, answering to 

 the shells of Bivalves, we select the genus 



Pyrosoma^ 



one of the second family of this division, con- 

 sisting of small animals, habitually united in a com- 



* Considered by some naturalists as a distinct Class, and named 

 TUNICATA. t n^, pyr, fire, and <?&/*<*, soma, a body 



