The Evolution of Vermetid Gastropods 
J. E. Morton 1 
Several families of prosobranch gastropods 
are preadapted, by the possession of efficient 
cleansing mechanisms-the cilia and mucous 
tracts of the pallial cavity-to adopt a method 
of ciliary feeding. It is thence a natural de- 
velopment to take on an immobile habit, 
either buried beneath the surface of soft sand 
or mud (as in the Turritellidae [Yonge, 1946] 
or the Struthiolariidae [Morton, 195 la]); or 
attached firmly to a hard substratum (Calyp- 
traeidae, Capulidae, and Vermetidae, [Yonge, 
1938]). The gastropods known collectively as 
the vermetids are one of the most specialised 
of the latter groups: they have altogether 
abandoned the regular coiling of the typical 
gastropod and the shell has become untwisted 
and vermiform, attached to, or embedded in 
the substratum. In previous papers (1951^, 
1951 b, 1951c, 1953) the present writer has 
dealt in some detail with the structure and 
systematics of the vermetids, and it has been 
demonstrated that the classically recognised 
family Vermetidae must be broken into two 
very distinct groups-the Vermetidae s. str. 
and the Siliquariidae. Each of these has been 
derived, probably separately, from a free- 
moving regularly-coiled prosobranch stock. 
It has been thought useful here to sum up in 
a short space the adaptive features of these 
1 Department of Zoology, Queen Mary College, 
University of London. Manuscript received November 
2 , 1953 . 
two families and to discuss the separate evo- 
lutionary trends they have followed. 
THE VERMETIDAE 
The first detailed accounts of the feeding 
habits or adaptive morphology of vermetids 
were given by Boettger (1930) with a de- 
scription of Serpulorbis gigas, followed by 
Yonge (1932) with a discussion of Vermetus 
novae -hollandiae of the Great Barrier Reef. 
Yonge concluded, on the basis of the widely 
different feeding mechanisms revealed in Ser- 
pulorbis gigas and Vermetus novae -hollandiae (see 
also Yonge and lies [1939]) that these two 
vermetids belonged to widely separate groups, 
and that "the taxonomy of the Vermetidae 
clearly requires revision in the light of these 
results." Yet in this series of genera, the re- 
sult of further work (Morton, op. cit.) on 
Serpulorbis and Novastoa and a fuller knowl- 
edge of Aletes (McGinitie and McGinitie, 
1949: 366) was to bring together a large range 
of forms into a single sequence and to show 
that the ciliary feeding in Vermetus novae- 
hollandiae and the mucous trap feeding of 
Serpulorbis gigas are the extreme forms in a 
single evolutionary process. 
Of the genera now included within the 
Vermetidae s. str. seven are dealt with here, 
namely Vermetus , Petaloconchus , Novastoa , Spiro- 
glyph us, Bivonia , Aletes , and Serpulorbis. Of the 
two examples selected from Serpulorbis , S. gigas 
3 
