12 
THE YOYAGE OF H.M.S. CHALLENGEE. 
Immediately behind the mouth-slit a continuous ring of the following structure is formed 
by the buccal wall : — Superiorly are various leaf-like glandular papillae, succeeded on each 
side by an inferior dense margin composed of the two muscular processes which become 
terminal in extrusion. A variously folded papillose region occurs beneath and completes the 
ring. The chief muscles are massed at the sides of the dense superior folds. The great 
ridges increase in size and muscularity as we proceed backward, and the wall beneath 
becomes more compact. The muscular fibres moving the dense lips are evidently important, 
and are probably the chief agents in the biting movements which the part performs in 
alimentation. The fibres form a complex transverse and longitudinal meshwork, the free 
surface of the part having a glandular structure. Passing inwards, another fold then 
occurs externally on each side, and the superior papillae occupy a larger space, while the 
region next them has its surface covered with pointed papillae, and the dense portion is 
confined to the ventral part of the massive plait. The wall of the buccal organ presents 
the following minute structure ; — Below the dorsal thin glandular portion the massive 
lateral wall is formed by an interwoven series of transverse and longitudinal fibres, the 
conical papillae being on the inner surface. Beneath is a dense fold of the hard pad, 
which in section has the inner part of its area filled with longitudinal fibres. The 
secondary fold below is now made up of interwoven longitudinal and oblique fibres, and 
this and the former plait are bound to the dense outer wall of the organ by strong 
muscular bands. Instead, therefore, of forming the upper part of the lateral wall, the 
dense portion now constitutes the lower, the two accessory folds (enveloped in a firm 
layer) meeting in the middle line beneath, while the papillae have disappeared from the 
wall outside them. Immediately behind, the fold on each side unites with its fellow, the 
larger superior mass being formed chiefly of longitudinal fibres within the glandular 
layer, and the inferior of a complex interlacement of longitudinal and transverse fibres, the 
latter forming spaces for the former. The upper lateral wall is boldly papillose, while the 
dorsal margin has a row of blunt papillse. The lower part of the organ gradually merges 
into the upper, with similar radiating and circular fibres, the whole (with the exception 
of the dorsal arc) forming a dense muscular tube. Beneath is a great retractor of longi- 
tudinal and vertical fibres. Internally the long papillse of the lateral surface are continued 
over the ventral curve. The outer wall of the rounded tube is composed of a well-marked 
series of longitudinal fibres, which gradually increase in thickness from the dorsal to the 
ventral median line. The papillse on the inner surface then have a tendency to form 
ridges, and the thin glandular wall is greatly extended dorsally; while the canal 
diminishes in calibre, assuming a uniform outline, the inner surface being covered with 
the glandular rugse and papillse resting on a coat of circular muscular fibres — 
externally bounded by the longitudinal layer. 
The whole muscular mechanism of the organ is suited for performing the functions of 
a pair of flesh}^ biting pads, ’for the extrusion and retraction of these in varying degrees. 
