Pvn H,i I'UitlS

MONTACUTA. 207
favoured with its company. An ally, if not a variety,
of the present species of Montacuta was described by
Recluz in the ( Revue de Zoologie' for 1844 under the
name of Erycina seminulum, and stated to have been
discovered by M. Caron in the Mediterranean on the
spines of different kinds of sea-urchins, principally
Echinus esculentus. I mention these cases to show that
it is not confined to S. purpureus, or the " purple-heart
urchin," as is commonly supposed.
The byssal threads by which this curious mollusk
attaches itself are exceedingly strong and coarse. Mr.
Clark observed it in active motion after he had sepa-
rated it, still adhering to the spines, from a Spatangus.
He savs, "When the animal marches, its foot is ex-
tended, and its rounded termination is instantly fixed
to the vase in which it is deposited ; then by the re-
tractor muscle it is drawn forward, making such rapid
progression as to cross a watch-glass in a minute, and on
the passage turns itself several times by a twist of the
foot from side to side/' In another description which
he has given of this operation, he uses the appropriate
to show the toil-
expression " iterumque, iterumque/'
some march of the little creature, which evidently would
have preferred remaining on the sea-egg from which it
was so unceremoniously dislodged to taking a long
walk across and round Mr. Clark's watch-glass, in a
vain search for fresh quarters of the accustomed sort.
The gills and green liver are visible through the shell
in some specimens which are more transparent than
others, the former crossing it diagonally. The shape
and position of the cartilage is very remarkable. Some-
times the shell is partly incrusted with a ferruginous
deposit. The number of fry, with their shells com-
pletely formed, which are found in some individuals, is