Vermetid Gastropods — MORTON 
13 
oped the ciliary feeding habit as an evolu- 
tionary climax feature as mentioned earlier in 
this paper. They all fall within the wide series 
of Mesogastropoda bearing crystalline styles 
or protostyles, and relying strongly upon the 
action of cilia and mucus in the functioning 
of both the pallial cavity and of the digestive 
tract; they are preadapted to give rise to cili- 
ary feeders. A similar potentiality separately 
realised in different groups is the development 
of an enzyme-carrying crystalline style (Gra- 
ham, 1938; Morton, 1952). A common struc- 
tural pattern and mode of functioning of the 
stomach is inherited by all of those molluscs, 
both gastropod and lamellibranch, derived 
from possessors of a protostyle. The enzymic 
action of the crystalline style was doubtless 
acquired separately within these groups in a 
number of parallel cases. 
The diagrams (Fig. 2a, b ) make sufficiently 
clear the differences in adaptations of the 
Vermetidae and Siliquariidae, both in the 
modifications developed in the foot of the 
adult and the pallial structures present in the 
embyro. A common ancestor among the mi- 
crophagous mesogastropods must have had 
the following structures, or the potentiality 
to develop them: (1) a crystalline style sac 
in the stomach, and the mucous string mode 
of food transport; (2) a pallial cavity with a 
well- developed gill equipped with strong cili- 
ated tracts for current-producing, cleansing 
and rejection, and (potentially) three longi- 
tudinal glandular tracts within the pallial 
cavity, a hypobranchial, basibranchial ("en- 
dostylar”) and infrabranchial (nuchal tract or 
food groove) ; (3) a mucous gland of the foot 
opening anteriorly to the sole, and potentially 
able to become very large; (4) a small, strong 
radula of the grasping type, with sharply 
pointed teeth; and (5) a foot bearing a simple, 
circular, saucer-shaped operculum. Obviously 
almost any family of microphagous meso- 
gastropods would satisfy these requirements, 
and there is very little to indicate that the 
Vermetidae and the Siliquariidae arose close 
together. 
SERPULORBIS 
(zelandicus) 
Fig. 4. Scheme to express the relationships and 
course of evolution of the vermiform gastropods in- 
cluded in the Vermetidae and the Siliquariidae. 
The evolution of the two series (Fig. 4) 
shows first a convergence in the parallel devel- 
opment of ciliary feeding, and then a swing- 
ing away of the vermetid line towards more 
highly specialised mucous feeding. On the 
one hand, ciliary feeding is continued, with 
increasing specialisation of the pallial organs 
(Siliquariidae), on the other, the pedal gland 
at an early stage assumes a share in the pro- 
vision of mucus for food collecting. The gill 
filaments in this series never lose their original 
triangular form, and in the furthest evolved 
members of the Vermetidae, the pedal gland 
is enormously enlarged and the gill tiny and 
vestigial. In the siliquariids the operculum 
becomes large and very elaborate; in the ver- 
metids it is finally lost altogether. Parallel 
with the enlargment of the pedal gland and 
