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 themselves. 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 ( Z¢reds).—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. 
Fic. 62.—Ship-Worm (Zeredo navalis). 
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 Zeredo gigantea, 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 
