GENERAL CHARACTERS. 325 



thrushes, and other birds ; by lizards, toads, snakes, and even by certain kinds of 



carnivorous insects. The fresh-water forms are consumed in vast quantities by 



water-birds of every description, by fishes, frogs, water-voles, and other mammals, 



and aquatic creatures of various kinds ; and every seashore is constantly ransacked 



by flocks of sea-fowl for the repasts of shell-fish it affords. Out in the depths 



of the ocean many kinds of fishes, especially cod, haddock, gurnard, soles, and 



mullet, are great devourers of molluscs, which ever fall a prey, not only to one 



another, but also to crabs, holothurians, sea-anemones, and star-fishes ; and, finally, 



among the pelagic pteropods the Greenland whale seeks his daily sustenance. 



Molluscs of all kinds, but especially the marine species, are much eaten by the 



natives of most countries ; and even in Europe, although the oyster is the most 



highly appreciated, several other species are used as food. Molluscs are not only 



of importance to man as an article of diet, but they are serviceable in other ways. 



Their shells are employed as personal ornaments, and are used in the manufacture 



of fishing-tackle by some uncivilised people. In England and other countries 



many of the pearly species are manufactured into ornaments and various useful 



articles, and the beautiful pearls themselves, secreted within the tissues of the 



pearl-oyster, are esteemed as jewels. 



The utility of the molluscs to man probably far outweighs the 

 Noxious Molluscs. . ... . 



injury which is occasioned by a few kinds. In the foremost rank of 



the noxious species stands the Teredo, the great destroyer of submerged timber. 

 The damage done to piers, boat-bottoms, and in fact to wood of any description 

 which is located in the sea, is enormous, and there seems to be no effectual means 

 of meeting the attack of these molluscs, except by covering the timber with metal- 

 sheeting. The stone - work of breakwaters occasionally becomes more or less 

 damaged by the burrowing habits of the Pholas and Saxicava. On land, snails 

 and slugs commit onslaughts upon our crops and gardens, but these pests are more 

 easily overcome than their marine relatives. 



Although this is a subject very fascinating to some, it is one 

 which pre-eminently opens the gates of speculation. That species 

 have certain geographical and bathymetrical limits in their distribution, may be 

 an admissible fact in very many cases, but when the reason for this limitation is 

 sought we are reminded how little we know of natural causes. That certain tracts 

 of coast have their own peculiar inhabitants, and that the molluscs of the eastern 

 shores of America, for example, differ from those of the west we must admit ; but 

 how this has come about, is matter of conjecture. We say that differences of 

 environment, of food and temperature, are sufficient reasons to account for such 

 things. On the contrary, we are met with the fact that certain species in a given 

 genus have a much wider range than others, and we are fain to ask how this is 

 brought about. The range of terrestrial molluscs is much more restricted than 

 that of most marine forms. This is readily understood, as the means of dispersal 

 are very different. The early stages of marine molluscs, if not free-swimming 

 creatures, are liable to be carried great distances by ocean currents, or the action of 

 the tides and wind. On the contrary, land-molluscs are creatures of slow progres- 

 sion, and are liable to have their distribution hindered, either by rivers, mountains, 

 or seas. Consequently we find that island faunas, as regards the terrestrial species, 



