ON THE HISTOLOGY OF THE PEDICELLARIA, 387 
when intact a fenestrated appearance. The soft portion of the stalk between 
the distal end of the rod and the bases of the valves is made up of an outer layer 
of epithelium, which has already been described, and within this a sheath of fibres, 
which enclose a central core of an amorphous granular substance containing 
nuclei here and there, especially near the centre, but in which no definite cell 
outlines are discernible (Pl. XX. fig. 9). 
The fibres are attached to the knob-like upper extremity of the calcareous 
rod below, and above they end in a very remarkable manner. The majority 
of the fibres are folded upon themselves just below the base of the calcareous 
valves, forming a series of loops, while at the two sides these bundles are 
prolonged much farther, and are wrapped round the semicircular arches of the 
calcareous valves, as shown in Pl. XX. fig. 4. This whole structure, when freed 
from its attachment to the valves by dissolving them away with chromic acid, 
is shown in fig. 5. The looped portion of the stalk fibres appear to be of the 
nature of elastic tissue, and at any rate are very unlike the rest of the fibre, 
being more highly refractive, with sharp outlines and clear central portion. 
The fibres of the stalk are of very varied diameter, and show a faint longitudinal 
striation. Their outlines are also somewhat indistinct ; no transverse striation 
has been shown to exist in these fibres, and they are, in fact, very unlike the 
muscular fibres of the other parts of the pedicellariz. It is probable that they 
are more of the nature of connective tissue, but this is of course a very 
difficult question to decide. Certain of the epithelial cells which have been 
already mentioned are continuous with delicate processes (Pl. XX., fig. 6), 
which run along the stalk. 
The head of the pedicellaria now alone remains to be dealt with. Each of 
the three valves (Pl. XX. fig. 1), which make up the calcareous framework, is 
concavo-convex. The convex portion pointing outwards, the outline is more 
or less oval, and the valves are articulated to each other by three sharp teeth, 
the margin of the upper part of the valve being also provided with fine den- 
ticulations. The lower part of the valve is prolonged into a semicircular arch 
like the handle of a jug. From the inner surface of the valve arises an apophysis 
at right angles to the plane of the valve, which serves for the attachment of 
the transverse muscles. The valve is perforated throughout its whole extent 
by numerous foramina. Treated with acid the calcareous skeleton is dissolved, 
leaving behind a granular organic basis. The soft parts of the head consist of 
(1) the outer epithelium already described, (2) the transverse muscles, forming 
three masses arranged in the form of a triangle, the angles of which are formed 
by the apophysis of the calcareous valves, to which the muscles are attached 
directly, without the intervention of anything analogous to tendon ; the fibres 
themselves are transversely striated (Pl. XX. fig. 12, a), the striation being visible 
only on treatment with certain reagents, and even then being often confined to 
