ORGANIC DEPENDENCE AND DISEASE 
21 
impenetrable and typically nn jointed armor . 1 Witli them 
is to be grouped the vast army of brachiopods which 
thronged the early seas of the earth, a group whose or- 
ganic station has been much debated, whose historic posi- 
tion and anatomy separate them too widely from the mol- 
lusks to justify speculations as to their degeneration or 
derivation from that stock. 
This form of protective covering represents almost the 
extreme of defensive personal armor; a complete adjust- 
ment accompanied by, or resulting in, a stabilized inheri- 
tance. All groups of the Mollusca have not permitted this 
development to go to so great an extreme as in the lamelli- 
branchs, or clams, for the snail and the nautilus travel about 
carrying their coiled shells with them, quick to withdraw 
into them whenever danger comes and often to close the 
door behind with a shelly plate or hardened skin. Squids 
and cuttlefish, late representatives of the nautilus stock, 
have followed a divergent path in this development by 
which their outer shell has been enfolded within the body 
substance. These creatures, too, maintain an active mo- 
bility, flying like darts through the ocean waters. Ptero- 
pods, a very ancient and active molluscan type, and the 
translucent scapliopods are the surface swimmers of the 
deep seas. Both carry light external shells and all these 
together seem to portray the result of long struggle against 
the general enchainment of their class and to typify in a 
measure what the Mollusca might all have been had not 
subjection of close encasement been sought or thrust upon 
them. Among them all, the most palpable change, progress 
and variation of expression are within the active groups. 
A very much less seclusive body-cover was developed by 
the great group of articulated animals, the Arthropods, 
represented by the shrimps, crabs, lobsters and insects. In 
i Except chiton and such multivalvular mollusks, whose articulated shell ap- 
pears to be a response to the coiling habit which the animal had in much the 
same degree as the trilobite and the sow bug. 
