420 W. E. RITTER AND M. E. JOHNSON 
10 15 20 25 30 55 40 4§ 50 55 60 65 70 75 BO 85 HW G5 20 30 40 50 60 70 80 90 100 110 
& = 
CBR BAS 2S esha ee eae eees 
Gat ylatawle iol salar tct alee ce 
pa ease ea aar alee 
oe Ly a i aw SAREE E 
) Mena ee PeRRRRST RR eee Sal Ss: | 
ces GEaWERe 
Fercentage of increment 
10-90 Unbroken part of chain 
20-110 Wheel portion of chain. 
2] 
i ila ie eal 
AC a PLA 
Jee 
Fig. 9 Percentage of increment throughout chain 
DISCUSSION OF THE OBSERVATIONS FROM THE CAUSAL STAND- 
POINT 
1. Cause of the twist 
Although salpae do not move through the water very rapidly, 
still there is enough motion to make the end of the chain double 
back under the parent, as soon as it projects into the water. The 
reason for its turning over is less evident. The zooids begin to 
pulsatesome time before the twist is reached and, having been with 
aboral extremities uppermost in the original or normal position, 
we may suppose they tend to assume the same position again when 
the normal state of things is interfered with by the bending 
back of the chain. Observation of the living animals shows that 
the chains of wheels and the separate wheels (at least the smaller 
ones) usually move along with aboral extremities uppermost. 
It is easier to say that zooids ‘tend to assume the normal posi- 
tion’ than to show the cause of this tendency. It may be that the 
specific gravity of the oral ends is greater, or that the pulsation 
may have something to do with it, or there may be some tropism 
