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giving the appearance of a foot that has been split. These flaps can 
be spread apart so that the two inner surfaces form an expanded disk. 
In 1853 ForBES and HAnuey, in describing Nucula nucleus in 
their History of British Mollusca and their Shells, made the following 
statement. “The foot is white, and as if pedunculated and deeply 
grooved, so as to expand into a broad leaf-shaped disk with serrated 
margins: by means of this organ it can creep like a Gasteropod, and 
we have seen it walk up the sides of a glass of sea-water.” This 
seems to be the only observation of the kind on record. 
Soon after this account was published, Woopwarp, in his Manual 
of Mollusca, refers to FORBES’ observation, and adds that the foot is 
a burrowing organ. 
That the foot is a burrowing organ cannot be doubted by anyone 
who has studied the living animals on their native mud. That it 
ever acts as a creeping organ seems very doubtful to me. 
The authors who have adopted the view that the foot functions 
as a creeping organ have, in nearly every case, had only preserved 
material to work upon and, perhaps, finding so many characters that 
seem to them to denote generalized structure, they have been over 
ready to explain the modification of the foot as a creeping disk. 
Some Lamellibranchs are able to pull themselves over smooth sur- 
faces by means of their foot, and even climb rather abruptly inclined 
planes, but my observations lead me to believe that the form and 
structure of foot found in this group is especially poorly adapted for 
such a purpose. 
Nucula delphinodonta, Fig. 1, a small form, found in Northern 
waters, possesses a comparatively large foot that executes the move- 
ments of burrowing quite slowly. By 
means of it, the almost spherical shell 
is easily turned from one side to the 
other, and it not infrequently happens 
that, for a space of a few seconds, the — 
shell remains wholly lifted from the 
bottom of the dish, balanced on the foot. — 
Fig. 1. Nucula delphinodonta seen from the 
right side, with the foot protruded. 
This can not be considered as an act of creeping, however, for the move- 
ments of the foot are exactly the same as are used in burrowing, and 
the shell remains balanced only for a short time and then topples over. 
So far as I have observed, the other forms are not capable of 
balancing themselves even for this short space of time. 
