22 ORGANIC DEPENDENCE AND DISEASE 



simplest expression such animals are constituted of a suc- 

 cession, from head to tail, of a series of transverse body 

 segments, each of which is to be 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-shelled, 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- 



