10 
the normal position of the parts, in front of 
the proboscis. At the sides of this cushion, 
excurrent grooves carry particles of detritus 
and rejected debris off the surface of the sole. 
The anterior lobe of the foot contains a nest 
of mucous glands in the embryo, which — 
far from becoming enlarged in the adult— 
appears to be lost almost entirely. The epi- 
thelial covering is, however, well-supplied 
with mucous glands which serve for the 
cleansing of the foot. The whole of the mu- 
cous supply for food collecting is produced 
in the pallial cavity, chiefly from the endostyle. 
In the embryo of Stephopoma the operculum 
is a simple chitinous disc, slightly concave, 
carried on the back of the still functional foot. 
In the adult it becomes very large, overlap- 
ping the border of the foot, and densely (se- 
tose. It consists of a spirally coiled chitinous 
band, each whorl fringed with long, branched 
setae, those on the final whorl the longest and 
extending far beyond the edge of the disc. 
The present writer has fully described else- 
where this operculum and the structure of its 
bristles (1951 b) and has discussed a possible 
adaptive explanation of its structure. When 
the bristles are in contact with the edge of the 
shell aperture they have a screening and filter- 
ing action which prevents larger particles en- 
tering the pallial cavity. The animal may 
apparently feed for considerable periods of 
time in this position with the disc of the 
operculum close against the aperture and the 
sweeping fringe of gill filaments not extended. 
Deep withdrawal into the tube is a feature 
of some of the siliquariids as well as of the 
true vermetids. In Stephopoma it is made pos- 
sible by the flexibility of the opercular bristles, 
which may be recurved upwards against the 
inner wall of the shell when the animal is fully 
retracted although their full spread is con- 
siderably wider than the aperture. Whether 
the adaptive explanation of opercular special- 
isation is the full one is rather doubtful. Quite 
possibly the elaborate form of the bristle 
crown, and especially the interspecific differ- 
ences in the bristles in Stephopoma (see the old 
PACIFIC SCIENCE, Vol. IX, January, 1955 
paper of Morch, 1861), is, as must be the 
structure of the operculum in Siliquaria , a 
non-adaptive specialisation— one of those ex- 
travagances of evolution which seem so often 
to recur among gastropods at the level of 
family and generic systematic characters. 
The second section of the Siliquariidae con- 
tains those wormlike gastropods truly as- 
signed to the genus Siliquaria (s. lat.). Here 
the shell may be long, slender, and corkscrew- 
shaped, delicate and translucent as in the 
subgenus Pyxipoma , or squat and much heavier 
as in the Agathirses series. In all cases, how- 
ever, its spiral shape is retained, and along the 
right side runs a long spiral slit (reduced in 
some cases to a row of holes) corresponding 
with an internal fissure along the exhalant side 
of the pallial cavity. Pyxipoma weldii , from 
New Zealand and Australian waters, was the 
only siliquariid studied alive. It corresponds 
very closely in organisation with Stephopoma , 
and the latter genus was placed alongside the 
fissured "vermetids” in the Siliquariidae by 
Morton (195 lb). Characters in common are 
the structure of the foot, its absence of pedal 
gland and tentacles, and the nature of the 
ctenidial filaments which are narrow and lin- 
ear, quite distinct in form from those of the 
Vermetidae. In Pyxipoma the supplementary 
mode of feeding by means of the "sweeping 
fringe” has not been observed, though the 
general arrangement of the pallial organs 
makes such a mechanism quite to be ex- 
pected. The food groove, as in Stephopoma , 
forms a deep ciliated gutter, with no great 
development of mucous glands. The "endo- 
style” along the gill axis is large and highly 
glandular. 
The fissured shell is of much less taxonomic 
importance in marking the limits of the sili- 
quariid group than has been hitherto sup- 
posed. The present writer proposed (195 lA) 
to take as the chief diagnostic feature of the 
family the embryo shell, which is one and one- 
half whorled, coiled in an almost plane spiral, 
with a wide, slightly effused trumpet mouth, 
marking it off clearly from the adult shell, 
