EXPERIMENTAL OBSERVATIONS ON PENNATULIOS. 467 
this species it is usually thickest at junction of the stalk and 
rachis, where it is quadrangular in cross section, and tapers 
gradually above and below, becoming* cylindrical as it 
proceeds upwards and downwards. It usually extends almost 
to the base of the stalk, but terminates in the rachis some 
little distance below the apex of the colony. It is rigid in the 
middle but extremely flexible at each end ; and in the con- 
tracted condition of preserved specimens the basal end is 
usually bent upwards to form a hook as in the drawing of 
Pennatula rubra (fig*. 13). The apical portion of the axis 
is also usually hooked when the colony is contracted, but to 
a less degree. 
In the specimens of Virgularia the axis in the stalk 
becomes extremely attenuated towards the base, tapering 
gradually to a fine needle-like point, as in fig. 12. In this 
specimen, however, the axis does not extend to the extreme 
base of the stalk, as, according to the accounts given by 
i\Iarshall and others, is usually the case in this genus. 
The unusual length of the stalk, its extreme tenuity and 
flexibility of the lower portion of the axis, may account to a 
considerable degree for the extraordinary powers of con- 
tractility for which this genus is remarkable. 
The axis of Pennatula rubra is larger and stronger than 
that of the more delicate species P. phospliorea, and the 
investing integument is correspondingly stronger and thicker. 
In Pteroeides and other robust forms the axis is corre- 
spondingly stout and strong (figs. 11 and 13), and is less 
pointed above and below. In a state of complete extension 
the axis is continued from the base to the extreme tip of the 
colony. It is rigid throughout the greater portion of its 
length, but in a state of contraction it is shortened by the 
two flexible ends becoming hooked and slightly twisted 
spirally (figs. 14 and 15), the hook being more pronounced 
at the basal end. The spiral twist of the two ends of the axis 
was also observed in a dissection of Pennatula phos- 
phor e a . 
