No. 381.] A HALF-CENTURY OF EVOLUTION. 641 
latter, whose organs have undergone such reduction by para- 
sitism, and some of which through disuse have totally dis- 
appeared, did not evolve until some time after the appearance 
of mollusks and fishes. 
When existence in these early plastic vermian forms was 
confined to boring in the mud and silt, the body became cylin- 
drical, as in some nemerteans, and in the threadworms ; some 
of the latter forms, boring into the mud, became parasites, 
entering the bodies of other animals which serve as their 
osts. 
At about this time certain worms, as the simple mechanical 
result perhaps of threading their way over or through the 
rough gravelly bottom, became segmented. The establishment 
of a segmented structure, brought about by the serpentine 
mode of progression in the direction of least resistance, resulted 
in the origination of a succession of levers. Following this 
annulated division of the dermo-muscular tube of worms was 
the serial or segmental arrangement of the internal organs, t.e., 
the nervous, excretory, reproductive and glandular, and, in a less 
degree, the circulatory system. 
In certain of these primitive protannelids, as the result per- 
haps of external stimuli intermittently applied, bristles origi- 
nated to aid in progression, and finally the segmentally arranged 
lateral flaps of the skin, the parapodia, which served as swim- 
ming organs. Other nepionic forms, at first free swimming, 
became fixed and protected by two valves, as in the Brachi- 
opoda, which owe their success in Precambrian times to their 
fixed and protected bodies. 
Not long after the annelid type became established, that 
of the echinoderms apparently diverged from some nepionic 
worm, like a trochosphere. In such a form there was a tend- 
ency to the deposition of particles and plates of lime in the 
walls of the body, and the type becoming fixed at the bottom, 
or at least nearly stationary, and.meanwhile more or less pro- 
tected by a calcareous armor, lost its originally bilateral, and 
acquired a radial symmetry. 
But no echinoderms have yet been detected in Precambrian 
rocks, which, however, have revealed arthropods, as shown by 
