BIVALVE MOLLUSC A. 32 1 



by the branchiae, is divisible into two sections, the Siphonida, from 

 the animals having respiratory siphons, and the Asiphonida, Avhich are 

 destitute of them. 



The genus Mya may be taken as a t}'pe 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 Pholadidse, 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 Teredo 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. By a strange 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. Quotrefages, 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- 



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