306 MARINE 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 only 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 
