422 W. E. RITTER AND M. E. JOHNSON 
involved; but we have no observations under this head. As the 
chain lies, with aboral ends of the zooids uppermost, the propul- 
sion of the zooids drives them away from the ventral side of the 
parent, while if they were with oral ends up the pulsations would 
drive the oral ends up against the ventral side of the parent. In 
one specimen in which the. chain had not yet emerged, the end of 
the chain was turning to get around the placental blood vessel. 
Had our observations been limited to this one instance, we might 
conclude that the twist is initiated in this way. But after look- 
ing over a large amount of material and finding that the placenta 
usually disappears before the chain reaches that point, it is evi- 
dent that this in no way accounts for the twist. 
2. Unequal growth of zooids and foot-preces as a factor in the 
breaking up of the chain 
We seem to have found a cause sufficient for the present re- 
search, for the break, 7n some way, of the chain of zooids. This is, 
as already pointed out, the unequal growth of the bodies and foot- 
pieces of the zooids. The first question that arises when we 
attempt to push the analysis farther is, why is the break into groups 
rather than into single pairs of zooids? Nothing in the differen- 
tial growth recognized appears to bear upon this question. So 
far as that is concerned we should suppose the zooids would be 
picked off one by one, or at most in single pairs. 
Just how constant these groups are, may be seen from the fre- 
quency polygon (fig. 10). We see that of ninety-two half wheels 
seventy-three contained six or seven zooids each, while only two 
contained eight, and five contained four zooids. This constancy 
is to be expected when we regard the breaking apart as a growth 
phenomenon depending upon constant causes rather than upon 
chance. 
In some way the wheel phenomenon is clearly dependent to a 
large extent on the strength of the adherence among the foot- 
pieces, which are but parts of the central ends of the peduncles. 
As may be seen by fig. 18, the radial blood vessels, the other 
main connection of the zooids, break apart early in the life of the 
