APR. 4, 1923 CLARK: ORIGIN OF VERTEBRATES 135 
Three such intermediates (fig. 3) seem to be clearly indicated in the 
polyzoans, colonial and not at all or very imperfectly coelomate, 
between the rotifers and the graptolites; the arthropods, with a 
segmented body like that of the annelids, but divided into two or three 
units showing division of labour (in the insects one controlling and 
directing, one locomotor, and one performing the digestive, reproduc- 
4th readjustment 
(7 
7 
a — 
3rd readjustment \""s<—99""" ee” ? 
lst readjustment 
2nd 
readjustment 
2 
Fig. 2. Showing the first to the fourth readjustments. 
tive and other vital functions) after the graptolite or polyzoan fashion, 
with a poorly developed coelome, with abundant traces of asexual 
reproduction (polyembryony, parthenogenesis, fragmentation of 
larvae, etc.), with a marked tendency to form (as in the ants) poly- 
zoan-like colonies with division of labour among the (dissociated) 
units, and sometimes even forming dendritic colonies (as in Thomp- 
sonia) ; and the molluscs, always solitary, like the priapulids and sipun- 
