BIVALVE MOLLUSCA. 321 
by the branchiz, is divisible into two sections, the Siphonida, from 
the animals having respiratory siphons, and the Asiphonida, which are 
destitute of them. 
The genus Mya may be taken as a type of the first, and the oyster 
of the second. The division Siphonida is divided into two sub- 
sections, those without and those with a pallial line sinuated. The 
first family of this latter section is the Pholadide, which includes the 
genera Teredo, Xylopagha, and Pholas, animals which possess extra- 
ordinary powers of boring, not merely through sand, but through the 
hardest rocks. 
The genus Zéredo consists of marine animals having a special and 
irresistible inclination for submerged wood ; for while wood exposed 
to the air becomes a prey to terrestrial animals, so submerged wood 
is subject to invasion by aquatic animals, of which the Teredo is by 
far the most formidable. The Teredo in the bosom of the ocean 
perforates the hardest timbers, whatever be their hardness. The 
galleries bored by these imperceptible miners riddle the whole interior 
of a piece of wood, destroying it entirely, without the slightest external 
indication of its ravages. The galleries sometimes follow the grain of 
the wood, sometimes they cut it at right angles; the miners, in fact, 
change their route the moment they meet in their way either the 
furrows hollowed out by one of their congeners, or some ancient and 
abandoned gallery. Byastrange kind of instinct, however multiplied 
may be their furrows or tubes in the same piece of wood, they never 
mingle—there is never any communication between them. The wood 
is thus attacked at a thousand diverse points, until it is invaded, and 
its entire substance destroyed. It is by secret ravages of this kind 
that the piles and other submarine constructions upon which bridges 
are built are often riddled and perforated. They appear to all out- 
ward examination as solid and perfect as at the moment they were 
first driven; but they yield to the least effort, bringing ruin and 
destruction on the edifices they support. Ships have been thus 
silently and secretly mined, until the planks crumbled into dust under 
the feet of the sailors. Others have gone down with their crews, 
their destruction being entirely caused by the ravages of these relent- 
less enemies, which are terrible from their unapproachable littleness. 
M. Quatrefages, who has minutely studied the organisation and 
habits of the Teredos in the Port of St. Sebastian, reports the following 
fact, which will give the reader some idea of the rapidity with which 
these dangerous molluscs pursue their ravages :— 
“A boat, which served as a passage-boat between two villages on 
the coast, went down in consequence of an accident at the commence- 
Vv 
