316 MARINE INVERTEBRATES 
thurian, or that poth of these have the closest family ties with 
the sea-urchins? 
The probable reason for these startling departures from a typi- 
cal form is that in the lower orders of life, where the organs and 
their functions are comparatively simple, the laws of evolution 
operate far more quickly in the adaptation of an organism to 
environment and changed conditions of life. A decided tendency 
to this departure from the type is a feature of the mollusks. It 
would be difficult, if not impossible, to give a general description 
that would fit both a common garden-snail and the common oyster, 
yet both are mollusks. A concise definition of the phylum is there- 
fore practically impossible, and it seems here as if nature resented 
all attempts to cireumscribe her living forms by rigid rules. 
The most persistent characteristic of the Mollusca is the posses- 
sion of the “mantle,” a sort of outer skin which, like a bag or sack, 
envelops the creature above and about the sides, but is open 
below, permitting the animal at will to protrude or withdraw its 
foot. From the outer surface of this sheath-like mantle, or fold, 
a calcareous shell is secreted. The modifications of this mantle 
are infinite; indeed, in some well-marked groups it seems to 
have wholly disappeared, having in reality become fused to the 
sides of the animal, and having become therefore a mere outer 
skin, and no longer a tunic or cloak, as originally intended. As 
already noted, the mantle does not always secrete a shell, a fact 
which again reminds us that we cannot give a good comprehen- 
sive description of the phylum that always holds true throughout 
its subdivisions. The best that can be done is to give in general 
terms the characters which appear in a majority of the forms. A 
convenient method of going about this is to create, for the sake of 
clearness, an ideal, model, or fanciful mollusk and clothe it with all 
the most salient molluscan features, and from such a basis to de- 
velop, as we go on, the various types of the actual orders and sub- 
orders. There is one class of mollusks, the Amphineura, which 
conforms reasonably well in most of its features to our compre- 
hension of the schematic mollusk. This class is sometimes referred 
to by biologists as the “primitive” or “original” mollusks, from 
which all the other classes have since been derived ; but the evolu- 
