8 
PACIFIC SCIENCE, Vol IX, January, 1955 
zelandicus, which have continued to make 
good use of the gill in feeding, mucous trap 
feeders like S. gigas, and mess -table feeders 
like Aletes squamigerus . The adult shells are 
markedly similar and devoid of diagnostic 
features. Evolution in sculptural detail has 
tended to become retrograde and is moreover 
often obscured by encrusting organisms or 
becomes obsolete on the older parts of the 
shell. The radulae in the above three species 
are extremely similar and thus unhelpful. The 
most natural means of separation would at 
present appear to be by reference to the con- 
dition of the gill and the ciliary tracts of the 
mantle cavity, which is of small help to the 
conchologist faced with a range of cabinet 
material. Perhaps the most valuable character 
will be found in the sculpture of the embry- 
onic shell; and here the careful work of Dr. 
Myra Keen on the American vermetids will 
be eagerly awaited. It is possible that the 
Aletes-Serpulorbis group should have the status 
of a single worldwide genus, when judged on 
shell characters; if this is so, it will be a genus 
in which evolution of the animal and its mode 
of life has greatly outrun the changes in con- 
chological features. 
THE SILIQUARIIDAE 
In the second family now recognised, the 
Siliquariidae, the shell never loses its roughly 
spiral general form; though as a general rule 
the spire loosens up to become open and 
corkscrew-shaped. The tube is never cemented 
to the substratum along the whole of one 
side, as in the most advanced and least regu- 
larly coiled members of the Vermetidae; it is 
usually, as in Stephopoma , loosely immersed 
in the encrusting growth on rocks, or, as 
often in Siliquaria , embedded in sponges. 
Mucous traps are not used in feeding and the 
pedal gland does not become greatly enlarged 
as it is in all the Vermetidae. In all cases the 
ctenidial filaments are employed in food col- 
lecting and they have become much more 
modified than the primitively triangular fila- 
ments of the vermetid type. The pedal tenta- 
cles are never present, and the sole of the foot, 
while reduced in extent, is usually clearly 
recognisable in the adult and considerably 
more conspicuous than in the Vermetidae. 
The account by Morton, (195 lA) describes 
in detail the mode of feeding of a member 
of the Siliquariidae, Stephopoma roseum. The 
more normal method of food collecting, with 
the gill retained in the pallial cavity, is clearly 
not very different from that of Turritella 
(Graham, 1938). The gill is also however 
employed in a "sweeping fringe” type of feed- 
ing activity, which has not been observed 
and recorded in any other gastropods. The 
gill filaments have become narrow and cirri - 
form, and lie across the pallial cavity to form 
an oblique lamina extending from its inser- 
tion on the left across to the food groove on 
the right. A continuous water current is drawn 
into the pallial cavity by the action of the 
lateral cilia of the ctenidium, and is passed 
between the filaments from the ventral (front- 
al) to the dorsal (abfrontal) side. Food parti- 
cles themselves, entangled in mucus secreted 
by the endostyle, are carried along the fila- 
ments to the right side of the pallial cavity, 
chiefly by the frontal cilia, to a lesser extent 
by the abfrontals. The "food groove” does 
not form a wide glandular sheet as in the 
Vermetidae, and correspondingly less mucus 
is derived from it. The greater part of the 
binding substance of the food string comes 
from the endostyle and, as in the Vermetidae 
also, the hypobranchial gland appears to con- 
tribute little if any mucus to the food string. 
The tips of the ctenidial filaments are rounded, 
somewhat expanded, and bulbous, and an- 
terior filaments crowd like a cluster of fingers 
into the wider portion of the food groove. 
During normal feeding, a number of cteni- 
dial filaments protrude across the free edge 
of the mantle, and when the animal is fully 
extended from its tube a fringe of radiating 
filaments forming about one-fourth to one- 
third of the total length of the gill projects 
from the mantle. Frequent sweeping move- 
