BRACHIOPODA. 203 



for the safety of Holland, from their destructive attacks upon the wood of the 

 flood-gates and dykes. A few weeks' immersion of a piece of fir-wood suffices 

 to enable the Teredo to bore it through and through, and even the hardest 

 oak is not able to resist this formidable destroyer. 



CHAPTER XVIII. 



FOURTH CLASS OF MOLLUSKS. 



BRACHIOPODA.* 



THIS is a very limited group, the members of which might 

 readily be supposed at first sight to belong to the ordinary 

 bivalves described in the last chapter. They are contained within 

 a pair of shells, more or less resembling those of the common 



FIG. 208. BRACIIIOPOD. 



cockle. One shell, however, is larger and more convex than the 

 other, and is generally pierced with a hole near the hinge. The 

 shells are for the most part fixed to some rock or other object by 

 a fleshy stalk, but in one genus (Orbicula) the lower valve itself 

 is cemented to the rock. 



On opening the shell the structure of the enclosed Mollusk is 

 at once seen to differ widely from that of the scallop and all the 

 Conchiferous class. On each side of the mouth, which is placed at 

 the bottom of the fold of the mantle, extends a fleshy arm, fringed 

 with long cilia. In some species these arms are of great length, 

 and can be protruded from the shells to a considerable distance, 



, brachion, an arm ; Trofs, TroSJs, pous, podos, afoot: arm-footed. 



