tooth (monostichous). It is a curious fact
that in the original type _Neomenia_ the radula is entirely absent, as
it likewise is in several genera of _Proneomeniidae_. The oesophagus
is short and leads into a long, straight stomach, provided with
numerous symmetrical lateral caeca. The stomach opens into a short
straight rectum which opens into the branchial chamber.

_Coelom, Gonads and Excretory Organs._--The coelom differs from that
of the Chitons in the fact that the cavities of the genital organs are
continuous with it, and in the fact that there is only one pair of
coelomoducts resembling the renal organs of Chitons, but serving also
as genital ducts. The gonads are paired and hermaphrodite, they form a
pair of anterior prolongations of the pericardium, extending nearly to
the anterior end of the body. Ova are developed on the median,
spermatozoa on the outer wall of each genital tube. The pericardium is
ciliated internally on its dorsal and lateral walls. The urino-genital
tubes arise from the posterior angles of the pericardium, pass first
forwards, then backwards, and unite to open by a common opening into
the cloaca below the anus except in _Strophomenia_, where the openings
are separate. Usually each tube is provided with caecal appendages on
its proximal portion, and these serve as vesiculae seminales, while
the distal portion is enlarged and glandular and secretes the
egg-shell.

_Heart and Vascular System._--There is a heart in the pericardium
consisting of a median ventricle attached, except in _Neomenia_, to
the dorsal wall of the pericardium, and in _Neomenia_ a pair of
auricular ducts returning blood from the gills to the ventricle. The
aorta is not independent as in Chitons, but is a sinus like the other
channels of the circulation. A single median ventral sinus passes
backwards to the gills or cloaca. The blood is coloured red by
haemoglobin in blood corpuscles.

_Nervous System._--Ganglionic enlargements are more conspicuous than
in the Chitons. In front of the buccal mass is a median cerebral
ganglion. From this pass off two pairs of cords, the pleural and
pedal, in _Proneomenia_ separate from their origin, in _Neomenia_
united at first and diverging at a pleural ganglion. The pedal cords
anteriorly form a pair of pedal ganglia united by a thick commissure.
The supra-rectal commissure may be present and bear an ovoid ganglion