6
the oysters which spawn in July or August, whereas the
barnacles as we have seen are capable of reproduction
almost the whole year.
Consequently we can take it for granted that the Wal
cheren oysters in October 1945, when their existence came
abruptly to an end, were between 3 and 4 months old.
In their turn several oyster shells were bearing barnacles
on their upper valves. The largest of these reached about
3 mm. in diameter. It is evident that these barnacles must be
younger that oysters, being at most 23 months old. Herewith
we have another demonstration of the extremely rapid deve
lopment of these creatures.
Of the bottom dwellers I could examine a shovelful of the
common cockle Cardium edule L.and about a dozen clams
(Mya arenaria L.)
The cockles varied between 27 and 14 mm length, and
2413 mm height, the Mya's between 54 and 37 mm length,
and 3522 mm height. Just like the mussels the cockles and
clams do not show winterrings.
The cockles have strong, equilateral shells. There is no
tendency to develop a produced posterior end as in the case
of animals living in brackish water. Like the Cardium the
Mya too are representatives of the high sea. None of them
show the fragile, stunted form which occurs in the Zuiderzee
or in the Baltic.
Cardium and Mya, although they are in fact not actually
sessile yet physiologically they can be regarded so, because
they live permanently imbedded in the mud, Mya never, and
Cardium rarely, quitting their milieu.
According to Havinga (Handb. Seefisch. Nordeuropas,
Bd. Ill, Heft 2, 1929, S. 108) Cardium edule breeds in Hol
land between February and October. After one year the length
amounts to 20 mm. Compared with the Walcheren speci
mens which are about 9 months old, it is evident that the
latter are none the worse for it.
On the development of Mya information is much more
incomplete. Breeding takes place in summer, larvae are found
in the plankton between June and September. The youngest
bottom stages crawl freely on the mud, or attach themselves
by means of delicate byssus threads to algae, hydroids and
other substrata. Not until a few weeks later are the clams
going to burrow in the mud, at first superficially, with increa
sing age deeper and deeper.
In my opinion the Walcheren clams are derived from young
