242 DIVISION II. MOLLUSCOUS ANIMALS.—CLASS IV. ACEPHALES. 
value, that the Republic of Venice once made a present of one to Francis 
I., who gave it to the Church of St. Sulpice, in Paris, where it is still 
made use of as a basin for holy water. The Tridacna attains a diameter 
of five feet, and a weight of five hundred pounds, the flesh alone weighing 
thirty. 
The muscular power is said to be so great as to be able to cut through a 
thick rope on closing the shell. It is found in the dead rocks on the coral 
reefs, where there are no growing lithophytes, except small tufts. Gener- 
ally only an inch or two in breadth of the ponderous shell is exposed to 
view, for the Tridacna, like the Pholas, has the power of sinking itself in 
the rock by removing the lime about it. Without some means like this of 
security, its habitation would inevitably be destroyed by the roaring break- 
ers. A tuft of byssus, however strong, would be a very imperfect security 
against the force of the sea for shells weighing from one to five hundred 
pounds. It is found in the Indian Ocean and the Pacific, as far as the 
coral zone extends. The animal of the Tridacna, and of the nearly related 
Bear’s-paw (Hippopus), distinguishes itself by the beauty of its colors. 
The mantle of the Zridacna safranea, for instance, has a dark-blue edge 
When a 
large number of these beautiful creatures expand the velvet brilliancy of 
with emerald-green spots, gradually passing into a light violet. 
their costly robes in the transparent waters, no flower-bed on earth can equal 
them in splendor. 
Proxas. — The Pholades have two principal valves, wide and ventricose, 
on the side ofthe mouth, narrowed and elongated on the opposite side, 
and leaving at each end a large oblique opening; the foot issues by the 
opening at the side of the mouth, which is the widest, and from the opposite 
end there come out the two tubes united in one, and capable of being ex- 
tended in every direction. 
The Pholades secrete a corrosive juice, capable of dissolving calcareous 
rocks. With the assistance of the secretion, and the action of its sharp- 
edged yalves, the pholas forms a pear-shaped cavern, in which it is con- 
demned to pass its whole life. The thicker part of the body, consisting 
principally of the very short but strong foot, fills the broad base of the 
hollow, while the long siphon is turned towards the narrow opening, from 
which it may be protruded at pleasure. All the movements of the animal 
are confined to a rising or falling in its narrow prison, 
Most of these animals are small, but some species attain a length of 
five inches. The fragile shell of the pholades seems to have prompted 
them to seek a better protection in hard stone. They are, consequently, 
noxious animals; they perforate the walls and calcareous jetties which 
man opposes to the sea, or raises for the creation of artificial harbors and 
