306 MAEINE INVERTEBRATES 



distinction of a phylum almost exclusively to themselves : they 

 retain their old associations only in the name of Molluscoida. 



With some exceptions, all mollusks secrete from their outer 

 skin, or mantle fold, a calcareous protective covering, or shell. 

 This may be either " univalve " or " bivalve " according as it con- 

 sists of one or two pieces. This phylum includes all the 

 sea-shells which are so commonly found along every ocean beach, in 

 the tide-pools, on rocks at low tide, in estuaries, and, indeed, 

 wherever sea-water is present. The phylum also includes, 

 as one of its large suborders, all the snails and slugs that are 

 to be found crawling upon the land. These are true mollusks, 

 which differ essentially from their marine brethren onty in that 

 they breathe by means of a pulmonary sac or lung instead of by 

 gills. There are also many genera and species of mollusks that 

 find their habitat only in the fresh water of rivers, lakes, or ponds ; 

 curiously enough, many of these fresh-water forms, like the 

 purely terrestrial snails and slugs, are air-breathers, possessing no 

 gills whatever, and are consequently obliged to make periodic 

 visits to the surface of the water to obtain their necessary supply 

 of oxygen. There are also numerous forms of mollusks that are 

 entirely deprived of a shell covering ; and, again, there are inter- 

 mediate types between these two extremes that produce only more 

 or less developed rudimentary shells. Notwithstanding these 

 variations in the matter of a shell covering, an important con- 

 sideration in this phylum, by reason of their anatomical features 

 these "naked" forms are mollusks quite as much as are those 

 that secrete the most highly developed tests. 



The marvelous beauty of sea-shells and tropical land-shells, 

 their almost infinite variety in form and coloring, has given to 

 them an interest among collectors that is very great. There are 

 many wonderful conchological collections in public museums and 

 in private cabinets. It would well repay the lover of beauty as 

 well as the more serious student of nature to examine carefully 

 such collections when opportunity offers, for nowhere in the realm 

 of nature can more exquisite coloring and modeling be found. The 

 fact that shells may be preserved for all time without the expense 

 and the vexations of preservative fluids has no doubt induced many 



