22 
ORGANIC DEPENDENCE AND DISEASE 
simplest expression sncli animals are constituted of a suc- 
cession, from head to tail, of a series of transverse body 
segments, each of which is to he interpreted as a somatic 
unit. Specialization among these somatic elements early 
appeared by adaptation to definite functions, and advance 
in specialization was followed by coalescence of the an- 
terior segments as we see them in the carapace of the lob- 
ster or the head of the trilobite. But regarding this type 
in its inception, we have to deal with a repetitive series of 
elementary like parts, comparable only to and probably 
derivable only from the ancestral segmented worms. The 
epithelium of this group was so vitalized that it could elim- 
inate from the water carbonate and phosphate salts of lime, 
combining with them a certain proportion of organic mat- 
ter which may have been in part derived from the epithe- 
lium itself. Thus we find the members of the group for the 
most part thin-slielled, with the shelly cover of plates deli- 
cately jointed one with another, so that the motion of the 
parts, except for such as are fused together, and of the ap- 
pendages of the parts, similarly covered and jointed, is in 
no way impeded. And in the normal expression of these 
creatures there is no impairment of locomotion. 
This is a protection by an exoskeleton which is a per- 
fectly advantageous adjustment, as it involves no interior 
constraint of organic function. The arthropods may put 
on an infinitude of shape and be found adapted to all media 
of life; they may present innumerable expressions of ex- 
treme degeneracy, subservience and adapted solid protec- 
tion acquired by boring or burial ; but in all these conditions 
the type of epidermal protection is not fundamentally 
altered. 
The external shell on any creature, whether snail or sol- 
dier, is then a structure which, in the idea and the inception, 
helps, not hinders, in the fight against untoward conditions. 
Kept in subjection to the high function of locomotion, it has 
accelerated and helped to ensure progress. Used intern- 
