Vermetid Gastropods — Morton 
11 
with which it is set in the same axis, in strong 
contrast to the condition in the Vermetidae. 
The general mode of coiling of the shell, 
which is never internally septate, as well as 
the form of the dentition, are also good dis- 
tinguishing characters of the Siliquariidae. 
The structure of the operculum, which is 
always prominently retained, is equally typ- 
ical; it consists always of a spirally coiled 
band, bearing a fringe of setae, elaborate in 
Stephopoma , much more simple in Pyxipoma. 
In the latter genus, the operculum is raised 
to form a tall dome, built up of a spirally 
rolled band, enclosing a core of triangular 
cells arranged about a central axis. The setae 
here are never elaborate, and the operculum 
fits tightly over the aperture so that the animal 
is unable to retreat deeply into its tube. 
The significance of the spiral shell fissure 
in the siliquariids is perhaps not yet fully 
understood; but it is undoubtedly in part an 
adaptation of the same type as the primitive 
shell fissure in many Archaeogastropoda, by 
which the exhalant water current is released. 
Such a feature is not of course a primitive one 
in the Siliquariidae, but rather a reacquired 
character in members of a highly specialised 
group. Extra efficiency of the exhalant water 
current is an obvious advantage in a siliquariid 
living, as is common, in turbid waters on a 
highly sedimented substratum; moreover, the 
siliquariids lack the protective straining me- 
chanism which may be provided by the long 
opercular bristles in Stephopoma . The presence 
of an exhalant slit is perhaps also of adaptive 
value in the sudden and rapid withdrawal of 
the animal, which frequently darts back into 
the tube and closes the operculum tightly over 
the aperture. No expulsion of water is possible 
between the bristles of an opercular fringe as 
in the case in Stephopoma. Such an explanation 
of the shell fissure appears the most probable 
one in Agathirses and those siliquariids which 
do not deeply embed themselves in the sub- 
stratum. Its presence makes much less sense 
in Pyxipoma which is in its habit more special- 
ised, spending its whole life embedded in a 
EXH CPS 
Fig. 3. Diagrammatic transverse sections to illustrate 
differences between the vermetid and siliquariid organ- 
isation of the pallial cavity, a, Serpulorbis (Vermetidae); 
b , Pyxipoma (Siliquariidae). The arrows show the direc- 
tion of the principal ciliary currents. These figures 
should be examined together with the diagrams of 
Figure 2. 
br, Brood pouch in Pyxipoma; CPS, egg capsule in 
Serpulorbis; EN, endostyle; EXH, exhalant chamber of 
pallial cavity; fil, gill filament with tract of lateral cilia 
indicated; f gr, food groove in Pyxipoma; f tr, food 
tract in Serpulorbis; HY, hypobranchial gland; INH, in- 
halant chamber of pallial cavity; oes, oesophagus; 
OS, osphradium; PE G, pedal gland; RM, rectum with 
faecal pellets; sh, shell; sl, exhalant slit in Pyxipoma; 
*tr,’ muscular region of "trunk” formed by columellar 
muscle flooring haemocoele. 
mass of sponge, as far as the extreme edge 
of the aperture. Any advantage once derived 
from an accessory exit from the pallial cavity 
would seem here to have been entirely sacri- 
