Vol.. 8, 1922 
BIOLOGY: A. H. CLARK 
221 
though many of them have secondarily become soUtary through the disinte- 
gration of the colony coupled with an increase in size of the individuals. 
Among the coelenterates the solitary habit is commonly derived from 
the colonial through the process of strobilization. The original polyp 
elongates and from it plate-like units (ephyrae) one by one separate off 
by transverse fission distally which develop into large free living adults. 
The original polyp which gives rise to the strobila is always at first 
bilaterally symmetrical, w4th two tentacles opposite each other. In the 
simpler coelenterates the gastrula becomes the mouth of the adult, but in 
the more specialized types a considerable part of the external surface 
about this opening is drawn into it during growth forming a gullet tube. 
Bearing these two facts in mind it is easy to reconstruct the path by 
which the so-called higher animals were evolved from the coelenterate 
type. 
Strobilization in the form characteristic of the coelenterates became 
modified by the retention of the original bilateral symmetry and its accen- 
tuation, coupled with a more or less vague differentiation into a dorsal 
and a ventral surface, and the retardation of the liberation of the units 
formed, which are not detached until after sexual maturity has been 
reached, and furthermore have to a considerable degree lost their individ- 
uality, sharing a common nervous and excretory system. This condition 
is represented by the tape-worms, at the present time occurring only as 
parasites. 
The next step was the further reduction in the individuality of the 
segments and their retention throughout life, resulting in the formation 
of a definite elongate jointed body composed of a series of similar seg- 
ments in which a dorsal and a ventral surface are clearly distinguishable. 
Such a condition is represented by the annelids. 
The unified body thus formed by the consolidation of what were origi- 
nally separate and distinct entities in an animal colony which have now 
quite lost their individuality and become mere segments was still further 
perfected by the grouping of these segments into three units, the head, 
most unified, controlling and directing, the thorax, less completely unified, 
locomotor, and the abdomen, largest and least unified, containing the vital 
organs, as seen in the insects, or by a marked tendency to compress the 
body within the compass of a single group of firmly consolidated units, as 
seen in the crustaceans. In both cases the consolidation of the body was 
accompanied by a marked increase in the differentiation of the dorsal and 
ventral surfaces, and an extension of the former at the expense of the latter. 
The body of the insects and crustaceans was thus derived from the colo- 
nial coelenterates (as represented by the strobila) through the cestode and 
annelid types by the reduction of the colonial units to segments with pro- 
gressively decreasing individuality accompanied by the modification of the 
