HETEROMORPHOSIS 175 
b) The main root and the adventitious roots of Aglao- 
phenia, when they do not come in contact with the surface of 
solid bodies, bend downward and continue to grow in this 
direction; while the stems bend upward, grow toward the 
zenith, and arise from the upper surface of a horizontally 
growing root. 
V. The following phenomena are of importance in the 
general physiology of animal growth: 
1. For the growth of the tentacles of Cerianthus, as for 
the growth of plant tissues, it is absolutely necessary that 
the hydrostatic pressure in the cells of the organ exceeds a 
certain amount (that the organ is turgescent). 
2. The growth of the roots of Aglaophenia, Sertularia, 
and other Hydrozoa occurs only in a small region near the 
tip of the roots as is the case in the analogous plant tissues. 
3. When the roots of Aglaophenia, Gonothyraea, Plumu- 
laria, and Sertularia become attached to solid bodies, they 
grow in length much more rapidly, and their absolute growth 
is much greater than when they are surrounded on all sides 
by water. This has already been demonstrated by Dalyell 
in other Hydrozoa. 
VI. Of the special results the following only may be 
mentioned: If a transverse incision is made into the body- 
wall of Cerianthus near the oral plate, only those tentacles 
situated above the cut lose their turgidity, while the remain- 
ing tentacles retain theirs. The turgidity can therefore not 
depend upon a contraction of the body-wall which forces 
water into the tentacles. 
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