12 Herbert W. Rand 
the hemispherical end wall of the stump merges into the lateral 
wall. These conditions support the view that the tissues of the 
hemispherical end are in a state of muscular contraction, the con- 
traction being greatest at the base of the nipple and approaching 
nil at the equator of the hemisphere. 
Thirdly and finally, the fact that the nipple, having once been 
formed, may disappear and reappear in the course of the alternate 
contractions and extensions of the tentacle clearly indicates the 
muscular nature of the closing process. In this connection the 
mechanism of contraction and expansion of tentacles must be 
considered. In a fully extended normal tentacle there is appar- 
ently complete relaxation of both the longitudinal and the circular 
muscle fibers. Increase in length is not necessarily at the expense 
of diameter as it is in Hydra or in the body of the earthworm. 
The explanation of this difference lies in the fact that in the earth- 
worm the fluid content of the body is practically constant, whereas 
in the actinian the contents of the tentacle may vary owing to its 
being merely an appendage whose lumen is in free communication 
with he much more voluminous gastro-vascular cavity. 
The fully extended tentacle is at its maximum diameter as well 
as at its maximum length. In this relaxed condition its disten- 
sion is secured by the gentle internal fluid pressure. Shortening 
of the tentacle results from contraction of the longitudinal fibers, 
the circular fibers apparently remaining relaxed, for, at least dur- 
ing moderate contractions, there is no perceptible change of diam- 
eter. But in extreme contraction there may be contraction of 
circular fibers also. In the extension of a tentacle the longitudinal 
fibers are relaxed and in the normal tentacle there is usually no 
obvious contraction of the circular fibers. But in a detached piece 
of tentacle or in a piece of tentacle ligated onto the hydrostatic 
tube there may often—not always—be seen a marked diminution 
of diameter as the tentacle elongates (see Figs. g and 14a’). In the 
ligated piece this circular contraction sometimes yielded to a little 
internal pressure, but sometimes it did not. The behavior of a 
normal tentacle indicates that the conditions of extension are, first, 
general relaxation of all muscle fibers, and, secondly, internal 
fluid pressure. Child (’04b) considers at length the importance of 
