

ACEPHALOUS MOLLUSCA. 321 



a man placed on his belly, who, stretching out one hand, seizes upon 

 some fixed object, and draws himself towards it. There is just this 

 difference, that the movement of the member in the mollusc is 

 altogether contractile. 



Authors have described more than 30,000 species of molluscs, so 

 that our space only permits us to describe a few families, or rather 

 types of families. 



The arrangement of bivalves now most generally adopted in 

 England is that of Woodward, as developed in the last edition of his 

 manual of the mollusca ; it is greatly based on that of Lamarck. We 

 have adopted his arrangement altered from a descending to an ascend- 

 ing scale of organisation. 



The Conchifera are divisible into two sections, Siphonida, from the 

 animals having respiratory siphons, and Asiphonida, destitute of them. 



The solen 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 section is the Pholadidae, which includes Teredo, Xylophaga, and 

 Pholas, animals which possess extraordinary powers of boring ; not 

 merely as the Solens do, through sand, but through the hardest rocks. 



The Teredos are 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 Teredos in the bosom of the ocean perforate the 

 hardest timbers, whatever be their essence. 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 ; some- 

 times they cut it at right angles ; the miners, in fact, change their 

 route the moment they meet in their way either the furrows followed 

 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 



Y 



