, 
458 MR. S. PACE ON THE ANATOMY OF THE 
supra-branchial chambers by meaus of two valvular folds of the 
mantle-edge. The ospbradium, which is characteristically darkly 
coloured, measures about 10 millim. in length; it is broad, and 
the folia of its right side are much the more strongly developed. 
The lamellz of the ctenidium are nearly equilateral. The terminal 
portions of the rectum and oviduct were missing from the speci- 
men, and the renal organ was too fragmentary for description. 
The hypobranchial gland appeared but little differentiated, and 
there were no indications of any coloured secretion. 
As will be seen from the figure (PI. 42. fig. 3), the introvert 
apparatus presents greater complexity than is usually met with 
ia the less highly specialized Prosobranchs *. The rhynchostome 
is a wide, circular opening at the centre of the terminal disk of 
the rostrum: it is supported upon a massive cartilaginous ring. 
This aperture leads into a spacious cylindrical chamber (Rh.’) 
occupying the whole interior of the rostrum. The walls of this 
rostral chamber are glandular and exceedingly muscular. Pos- 
teriorly it is bounded by a thick septum whose central opening is 
entirely closable by means of a powerful sphincter. Succeeding 
this is another shorter chamber (Rh.") of the same calibre as 
the first, but having its walls quite thin and membranous. The 
lumen of this post-rostra] chamber was completely filled by the 
contracted-up proboscis (I.). Regarding the homology of these 
two prestomial chambers, it is probable that they together 
represent the simple rhynchodeum of the Rhachiglossa, rather 
than that tne anterior cavity 1s to be regarded as being pre-rhyn- 
chostomial, or the posterior one as an additional development. 
The introvert is a large, fleshy, tapering structure with a very 
minute external orifice at its extremity. Its interior is quite 
spongy by reason of the great development of innumerable cross- 
folds of the inner wall which almost obliterate its lumen. The 
plication appears to be very irregular, although with a certain 
tendency towards a spiral disposition. The alimentary canal is 
of wide calibre throughout its simple U-shaped course. The. 
pharyngeal bulb (Pl. 42. fig. 4, B.M.), which opens into the 
proboscis by a wide circular opening, is short and somewhat 
pear-shaped ; its walls are very massive and are supported upon 
cartilage ; the interior is finely plicated longitudinally, the plice 
* Tt is highly probable that further research will prove that the relationships 
of the introvert among the Toxoglossa are of an even higher order of complexity 
than is at present supposed, 
