OR FRESH WATER POLYZOA. 185 
Cristatella is no exception in the animal kingdom; 
there are many instances in which compound animals 
move and act in unity. But here there is some hope of 
solving this mysterious diversity of number, with unity 
of will and purpose. 
The nervous system, wherever it is present, whether in 
the distinct form of brain, nerve-mass, or ganglion, is es- 
sentially the medium of sensation and of motive power. 
Now if the nervous system among the Polyzoa is a 
compound system, having a common trunk with branches 
leading off into each Polyzoén, a sensation in the main 
body could be conveyed to each individual, and thus the 
will of every minute tube be brought into harmony with 
all, causing the whole to move like one creature. 
Fritz Müller, a German naturalist, has actually ascer- 
tained that in one of the marine species of Seriolaria, the 
nerves followed up the hollow trunk and branches of the 
colony like the dark wood in the heart of a tree, supply- 
ing each animal with a nerve. He noticed that if the 
trunk of the colony was irritated, that all the Polyzoa 
withdrew their plumes as if alarmed, and this led him to 
investigations, which resulted. in such important discov- 
eriés. 
Whether all the polypides in a colony of Cristatella 
unanimously resolve to move, or whether the majority 
rule and, drag the minority at will, or whether again the 
desire to move is excited in the central nerve-trunk by 
external causes, has not yet been determined. 
_ One thing, however, seems probable, that the unanim- 
ity of action in the little republic is due to the union of 
various individualized nervules into branches, and 
finally into one grand trunk, otherwise parts of the mov- 
able sac might be travelling in opposite directions at the 
AMERICAN NAT., VOL. I. 24 
