BIVALVES WITH SIPHONS. 



59 



except at the tips. They are found in nearly all oceans, 

 about 80 living species and 50 fossil being known. They 

 are noted for their boring powers (Fig. 61), making their 

 way into the hardest rock. 



NOTE. The shells contain aragonite in their composition, and are 

 supposed by continual friction to wear away the hardest rock. Having 

 entered a block of gneiss, they grow and enlarge their hole, and so im- 

 prison themseb'es. The pillars of the temple of Serapis, in Italy, are 

 pierced by them, showing that the land was submerged long enough 

 for them to obtain a foot-hold. The Pholas emits a bluish-white light 

 when dead or alive, and they may be compared to miners with their 

 lamps. A single one placed in seven ounces of milk has been used as 

 a lamp ; faces near it were illumined, and the milk appeared trans- 

 parent. Another, placed in honey, retained its light over a year. 



Ship- Worm ( Teredo). The shell is composed of two 

 curved equal valves, open at both ends (Fig. 62). The 

 animal is worm-like, and not entirely covered by the shell. 



FlG. 62. Ship- Worm (Teredo naval fs). 



They bore into wood, incasing the tunnel with a limy 

 ceiling. The siphons are long, and where they separate 

 are protected with small calcareous bodies, called palettes, 

 which close the mouth of the tube. The foot forms a 

 sucker. The common teredo is at first free swimming, 

 passing through several changes before it assumes the 

 adult form. The Teredo gtgantea, found at Sumatra buried 

 in the mud, is from four to six feet in length, the tube 

 three to four inches in diameter, and when covered with 

 other shells is extremely bulky and heavy. 



NOTE. The destruction they cause seems incredible. A large 

 portion of a wreck stranded on the Florida reef, and examined by the 



