388 MR PATRICK GEDDES AND MR FRANK E, BEDDARD 
comparatively few of the individual fibres making up the muscle masses, 
though sometimes all show striation. Treatment with chromic acid, KLEINEN- 
BERG'S picric acid, osmic acid, and in one case gold chloride, showed the striation 
beautifully, while alcohol was entirely ineffective in that direction (Pl. XX. 
fig. 12, e). Occasionally on treatment with these reagents, parts of many of 
the fibres swelled out into a spindle-shaped mass (PI. XX. fig. 12, d), continuous 
at both ends with unaltered fibre ; in this case no striation was ever observed. 
Rupturing the fibres showed that they were made up of an outer structureless 
sarcolemma and an inner muscular substance (Pl. XX. fig. 12, b). 
The head also contains three hitherto undescribed pseudo-skeletal structures, 
of extremely curious structure, and of remarkable beauty (Pl. XX. figs. 10, 11). 
These, from their highly refractile character, sharp edges, and resistance to 
action of dilute acetic acid, appear to be of the nature of elastic tissue. Each 
of these masses is flat, and formed of a number of fibres folded and re-folded 
upon themselves, forming a complex series of loops and meshes. These struc- 
tures are not to be confounded with the looped extremities of the figures of 
the stalk, and a glance at fig. 13, which represents a section through the head 
of one of these ophiocephalous pedicellarize will show that the two structures 
are entirely distinct. The same figure shows the position of these structures. 
Each is situated outside the muscular mass of its own side, but between and 
in apparent contact with the calcareous valves, and alternating with them. 
Moreover, they lie transversely to the long axis of the pedicellaria, while the 
looped termination of the stalk fibres lies parallel to the same axis. No con- 
nection with any soft parts either of the head or the stalk has been observed 
by us, and it seems very possible that their function is analogous to that of the 
elastic hinge in the shells of Lamellibranchs, serving to divaricate the calcareous 
valves. This view is supported by the fact that neither by sections nor by teas- 
ing can any connection be made out between these plates of elastic tissue and 
any other structure, and perhaps also would account for the imperfectly (7?) 
developed muscles of the stalk, their function, which would be presumably that 
of divaricating valves, being assumed by the mechanical action of these elastic 
structures. It is true that pedicellariz, when killed by being placed in reagents, 
immediately close, but this may be accounted for by a rigid contraction of the 
approximator muscles. 
The tridactyl pedicellariz need not detain us long, as they are not in their 
structure very far removed from the variety that has just been discussed. 
The three calcareous valves (Pl. XX. fig. 15), however, are of a very different 
shape; the main part of each valve is a tapering rod, which is pierced by 
foramina, and furnished at both edges for a considerable distance downwards 
with minute denticulations ; the basal portion of valve is triangular with teeth 
at the side angles for articulation with the other valves ; from the middle line 
a 
