332, W. A. HASWELL 
The only further mention is under Syllis hyalina 
(p. 227): ° Les diaphragmes transversaux ont la méme disposi- 
tion et la méme structure, a part ce fait que le tissu fibrillaire 
qui les compose présente vers la périphérie un arrangement 
en faisceau trés marque.’ 
But these annular muscles, as they may be termed, are of 
much greater importance than such casual mention as that 
given above would imply. 
Hach annular muscle is a bundle of non-striated fibres, com- 
pressed in the antero-posterior direction, running (in the 
prevailing third type) transversely between two adjoining 
rows of radial striated fibres in the immediate neighbourhood 
of the raphes, and, farther on, passing through the outer ends 
of the radial fibres. At the raphe the annular muscle is con- 
tinued straight across the middle lne to the opposite side. 
From the raphe the muscle runs in an annular way in the 
position indicated above, and is inserted at intervals imto 
the outer fibrous membrane. These insertions occur between 
the radial fibres of the row, around the corresponding 
accessory fibres (non-striated radial fibres) described below. 
It will thus be seen that the annular muscles are so arranged 
as to form a system of constrictors by means of which the 
lumen of the proventriculus, dilated by the action of the radial 
fibres, 1s contracted. 
The striated fibres, though the most important, are not the 
only radial fibres in the wall of the organ. Another set of 
radial elements, hitherto entirely overlooked, play a part 
which must be of some consequence, since their occurrence 
seems to be universal, and their arrangement varies little. 
These elements, which for the sake of distinction may be 
called the accessory or non-striated radial fibres, 
like the striated, run from the outer fibrous membrane to the 
inner. ‘They are single fibres (usually bifurcated close to the 
outer endin 8. variegata, usually branched in $8. corus- 
cans), placed at regular intervals between the striated fibres, 
us shown in figs. 1 to 4. As mentioned above, the main rela- 
tions of these fibres are with the annular strands of non-striated 
