Pvn H,i I'UitlS

208 KELLIID.E.
astonishing. Many hundreds of them, packed
close to-
gether, and glittering like microscopic pearls, might be
counted. They occupy at least two-thirds of the space
enclosed within the valves of their parent ; and its own
body seems to be atrophied
and dwindled to a mere
skeleton. The shell is in fact turned into a crowded
nursery. Perhaps the parent dies, like some insects,
immediately after all its progeny have been developed.
I do not concur in the general belief that M. substriata
is parasitic.
In one sense only can it be said to live on
Echinoderms. The food of Spatangus purpureus, on
which it is usually found, appears
to consist of ani-
malcula ; and for that purpose it swallows large quan-
tities of shell-sand, causing thereby a strong and fre-
quent current in the neighbourhood
of its mouth. The
Montacuta probably avails itself of this indraught, and
partakes of the sustenance intended for the Spatangus,
tube or
placing itself in the way, with its alimentary
opening turned in the right direction. No exudation
and its excretions
from the Spatangus has been noticed ;
would scarcely be produced in sufficient quantity for
the support of the Montacuta, or perhaps be suitable to
it. The latter has no suctorial organ, such as is pos-
sessed by all animal parasites : it never attaches itself
to the pedicellarise or any other soft part of the echino-
derm; nor has it once been detected on the back or
sides, or elsewhere than in the ventral region,
of its
associate. It is only found on the spines close to their
points.
2. M. bidenta'ta *, Montagu.
Mya bidentata, Mont. Test. Brit. p. 44, tab. 26. f. 5. Montacuta bidentata,
F. & H. ii.
p. 75, pi.
xviii. f. 6 & 6 a.
Bout clear white : mantle having a bluish tinge ; its margins
* Two-toothed.