9. THE CLASS CRUSTACEA
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walking legs slender, subcylindrical, last two articles with thin fringe
of hair; second and third legs subequal in length but with first leg
slightly stouter ; abdomen large, extending beyond carapace in all
directions.
Hard-shell male (Fig. 213) and female dark or brownish with two
large almost circular pale white spots on both carapace and sternum ;
with dorsal spots on brachial regions and ventral spots flanking abdomen
and medial to first pair of legs ; with well-calcified carapace, 1.4-4.6 mm
wide in males, 1.3-2.7 mm wide in females, flat dorsally, subcircular in
outline, with truncate front more advanced than in mature female ;
posterior margin straight ; lateral margin thin, sharply bent from dorsal
side, margin marked by a raised band of short dense hair ; eyes well
developed ; buccal mass crescentic, arched, broad from side to side but
short anteroposteriorly ; cavity completely closed by external maxillipeds; chelipeds stout; merus and carpus not slender as in mature
female ; palm slightly flattened inside, swollen outside, with both
margins convex ; hands with bands of pubescence on upper and outer
surfaces of palm and outer surface of immovable finger ; fingers stout,
toothed proximally, tooth fitting between two protuberances on immovable finger when closed; both fingers with stiff hairs on gripping
edges ; walking legs flattened, with posterior margin thickened and
with plumose swimming hairs on second and third pairs; abdomen
narrow, confined to sternal depression ; copulatory stylets of male
well developed, first pair blade-like and hairy, second pair rodlike and
almost hairless.
Life cycle. The pre-symbiotic larval stages of P. ostreurn occur in
the plankton. These include four zoeae followed by one megalops.
Descriptions of the first zoeal stages have been given by Birge (1882)
and redescribed by Hyman (1924) while descriptions of all the planktonic stages have been given by Sandoz and Hopkins (1947). The
planktonic larval stages do not possess dorsal or lateral spines on the
carapace. The time required from hatching to molting of the megalops
to give rise to the first crab stage is 25 days.
The symbiotic (parasitic?) stages have been studied by Stauber
(1945) and Christensen and McDermott (1958). The so-called '' invasive
stage '' is the first crab stage (Figs. 214 and 215). It is this form that
invades oysters. Six additional developmental stages ensue in the case
of females, including the mature adult stage. Males do not develop
beyond the hard stage. In both sexes it is the hard stage which is the
specialized stage a t which copulation occurs. The males leave their
hosts a t this stage to search for females in another host. It is only
during this stage that the crab is free-swimming. After copulating
319
walking legs slender, subcylindrical, last two articles with thin fringe
of hair; second and third legs subequal in length but with first leg
slightly stouter ; abdomen large, extending beyond carapace in all
directions.
Hard-shell male (Fig. 213) and female dark or brownish with two
large almost circular pale white spots on both carapace and sternum ;
with dorsal spots on brachial regions and ventral spots flanking abdomen
and medial to first pair of legs ; with well-calcified carapace, 1.4-4.6 mm
wide in males, 1.3-2.7 mm wide in females, flat dorsally, subcircular in
outline, with truncate front more advanced than in mature female ;
posterior margin straight ; lateral margin thin, sharply bent from dorsal
side, margin marked by a raised band of short dense hair ; eyes well
developed ; buccal mass crescentic, arched, broad from side to side but
short anteroposteriorly ; cavity completely closed by external maxillipeds; chelipeds stout; merus and carpus not slender as in mature
female ; palm slightly flattened inside, swollen outside, with both
margins convex ; hands with bands of pubescence on upper and outer
surfaces of palm and outer surface of immovable finger ; fingers stout,
toothed proximally, tooth fitting between two protuberances on immovable finger when closed; both fingers with stiff hairs on gripping
edges ; walking legs flattened, with posterior margin thickened and
with plumose swimming hairs on second and third pairs; abdomen
narrow, confined to sternal depression ; copulatory stylets of male
well developed, first pair blade-like and hairy, second pair rodlike and
almost hairless.
Life cycle. The pre-symbiotic larval stages of P. ostreurn occur in
the plankton. These include four zoeae followed by one megalops.
Descriptions of the first zoeal stages have been given by Birge (1882)
and redescribed by Hyman (1924) while descriptions of all the planktonic stages have been given by Sandoz and Hopkins (1947). The
planktonic larval stages do not possess dorsal or lateral spines on the
carapace. The time required from hatching to molting of the megalops
to give rise to the first crab stage is 25 days.
The symbiotic (parasitic?) stages have been studied by Stauber
(1945) and Christensen and McDermott (1958). The so-called '' invasive
stage '' is the first crab stage (Figs. 214 and 215). It is this form that
invades oysters. Six additional developmental stages ensue in the case
of females, including the mature adult stage. Males do not develop
beyond the hard stage. In both sexes it is the hard stage which is the
specialized stage a t which copulation occurs. The males leave their
hosts a t this stage to search for females in another host. It is only
during this stage that the crab is free-swimming. After copulating