EXPERIMENTAL ORSERVATIONS ON PENNATULIDS. 469 
supporting, extending and contracting the flexible apical 
portion of the axis. By the co-operation of these muscles 
with the longitudinal muscles of the body-wall the length of 
rachis would be materially increased or diminished. 
The somewhat oblique insertion of the apical muscles, 
coupled with the action of the transverse muscles of the body- 
wall, although less well developed in this region than in the 
stalk, would nevertheless induce a slight muscular dilatation 
and contraction of the central canals of the rachis. 
The spiral twisting of the apical portion of the axis and its 
ensheathing musculatiu'e, so noticeable in Pteroeides (fig. 
15) in a state of contraction, may possibly serve the purpose 
of a spring, which when released by the protraction of the 
muscles would tend to propel the colony in an upward 
vertical direction or vice versa. 
The oblique musculature is extremely well developed 
in Pteroeides and other large and robust Pennatulids (fig. 
14, Oh.M.). 
It consists of a series of “ obliquely set muscles-fibres ” 
whose direction is mainly longitudinal. T’hey are attached 
internally to the integument of the axis in such a manner 
that its basal portion is completely enclosed by a muscular 
sheath (figs. 11, 12, 13, 14), and are attached externally to the 
tissues lining the body-wall. 
Many of these muscles-fibres are embedded throughout the 
greater jjortion of their course in the tissues of the vertical 
se])turn. In Pteroeides the fibres are extremely stout and 
strong (fig. 14, M.A.A.). Those of the upper portion of the 
musculature are attached superiorly to the extremely stout, 
obliquely set “muscle bands” (fig. 14, M.B.) situated 
immediately below the sphincter muscle, with which it is 
also connected by obliquely set fibres. The muscle-bands are 
attached internally to the integument of the axis and to the 
tissues lining the body-wall ; the plane of their direction is 
slightly inclined to the horizontal. 
In the presence of the powerful “sphincter muscle 
and the two stout obliquely set muscle-bands” sup- 
