356 INVERTEBRATA OF MASSACHUSETTS. 



shoals at their mouths to such an extent as materially to diminish the 

 outlet for the waters, which then would rise and overflow their banks, 

 were it not for the destructive boring of the Limnoria. What could 

 not be accomplished by brute force, and might baffle even the ingenu- 

 ity of man himself, is yet quietly accomplished by the gradual and 

 steady operations of a tiny crab. The 'trees are perforated and then 

 washed away, and with them the sand and mud which had collected 

 around, and which would speedily have formed an effectual impedi- 

 ment to the free efflux of water, and to the navigation of the river." 

 These remarks have full force when applied to the mouths of most 

 of the rivers emptying into the Atlantic. " Nor must we omit to no- 

 tice the possibility of its being the means, occasionally, of causing the 

 removal of those serious obstacles to the safe navigation of shallow 

 seas, which are caused by the masts of sunken vessels." * 



If we except the various barnacles, which attach themselves to the 

 bottoms of vessels, sometimes in such numbers as materially to impede 

 their speed, I know of no other animal, coming within our limits, which 

 is either injurious or even disagreeable to man, in any sense. In many, 

 if not in all, parts of Europe, the snails and slugs are exceedingly 

 troublesome in gardens. But in this region snails are rarely seen, 

 and, in addition, the habits of American snails seem to be different 

 from those of European ones. They do not climb plants and shrubs 

 for food, and they mostly flee from the haunts .of men. Slugs are 

 also rare, and though occasionally seen in the vegetable garden and 

 the orchard, are not essentially troublesome. 



The invertebrate animals which may be noticed as beneficial, are 

 also few in number. They may be mostly arranged under 1. Such 

 as contribute food to man : 2. Such as afford subsistence to other ani- 

 mals : 3. Such as are employed in agriculture and the arts. 



1. Those which contribute food to man. The article of food which 

 is most familiar as well as most important among the Mollusca, is the 

 Oyster ( O'strea). 



The OYSTER is generally supposed by dealers and market-men to be 

 not now a native of the waters of this State. That it was formerly of 

 native growth, they all admit; but that it has become extinct in places 

 now most favorable to its growth, they all assert. The cause of this 

 extermination is said to have been what is called a ground frost ; that 



* Edinburgh New Philosophical Journal, xvi. 332, 333. 



