1884.] Colonial Organisms. 241 
which becomes the anal region of the anterior worm, and the 
other develops into the head and several succeeding segments of 
the posterior worm. The anal region may, by growth and seg- 
mentation, give rise to an indefinite number of new segments. In 
the fresh-water worms, the Nais and Chetogaster, this phenome- 
non proceeds farther. After the division into two worms and 
subsequent growth by segmentation, fission again and again 
takes place, whenever four or five new segments are added in 
front of an anal zone, so that a chain of zodids is formed, each 
with a few segments only and all fed from the mouth of the ante- 
rior one, Subsequently they break up into free individuals, each 
of which lengthens by budding. 
This phenomenon strongly indicates an original possession of 
the full life functions by a single segment which formed, by asex- 
ual reproduction, a longitudinal colony. Each member of this 
colony, while subordinated in function, tends to complete devel- 
opment, as is shown by the fact that certain segments apparently 
regain lost organs and resume discarded functions. The seem- 
ing indications is that these organs and functions were part of 
their original life plan, and that their ordinary development is 
only a partial one. 
In fact, in all these cases of individuals developed from colo- 
nies, the segments or organs present certain analogies to the lar- 
val state of other animals. The larva is an instance of retarded 
development in which the surrounding conditions check for a 
time the further unfoldment of the life plan, and cause the assump- 
tion of new temporary organs or functions. In the segment the 
unfoldment of the life plan is in like manner checked, and occa- 
sonally new organs or functions are developed to bring the 
organism into harmony with surrounding conditions, Ordinarily 
larva enters into new conditions and resumes its checked de- 
a t. Ordinarily the segment fails to enter into new condi- 
; tions, and its development continues partial. Yet there are inter- 
A Taa AAA in which the larval retardation continues for life, 
Leptocephali, or-embryonic fish. ~ And'to complete the 
Selig we have the above interesting instance of the Cheto- 
4 = TN hich the developmental energies of the segments .ap- 
ae | Sccasionally to triumph over the retarding conditions, and 
TEES ie ' 
Theia on * Growth and Development,” AMER. NATURALIST, July, 1883, P- 
YOU. xvns. nD, rrr, 6 
