9. THE CLASS CRUSTACEA
329
only slightly wider in relation to the width of the thorax than that of
the thick-shelled male. . . thin-shelled males (measure) 1-9-6.3 mm.”
Life cycle. P. pisum, like P. ostreum, includes four zoeal stages and
a single megalops. The first zoea has been described, but not very
effectively, by Thompson (1835) and later redescribed in detail by
Lebour (1928a,b). What may have been the second zoea has been
described by Lebour (19288). All of the stages, including the prezoea,
first through fourth zoeae, and megalops, have been described in detail
by Atkins (1955). The zoeal and megalopal stages are planktonic.
The most recent and complete accounts of the life cycle of P.
pisum are those given by Christensen (1959, 1962). Prior to this,
Thompson (1935) had briefly described male and female adults. He
stated that the males are hard-shelled while the females are soft-shelled.
This concept was held until Orton (1920) demonstrated the existence
of hard-shelled females which resemble males in all respects except for
their genital apertures and pleopods. Atkins (1926) has described the
molting stages of P. pisum in Mytilus edulis but followed Orton (1920)
in designating the hard-shell stage as the first crab stage (which
obviously does not correspond to the first crab stage of P. ostreum as
given by Christensen and McDermott (1958)). Later Atkins (1954,
1955) realized her error and abandoned the concept that the hard stage
is the first crab or invasive stage. Atkins (1955) has described the postembryonic development of P. pisum and still later (1958) has recorded
the occurrence of thin-shelled males which resembles stage I1 females.
Actually soft-shelled males had been found by Mercier and Poisson
(1929) but these French workers failed to realize that this is a normal
form. They were of the opinion that the soft-shelled condition had
resulted from parasitism by an entoniscid parasite. Although Atkins
(1958) has advanced our understanding of the stages in the life cycle
of P. pisum by definitely demonstrating the soft-shelled males, she
did not grasp their significance. She expressed the opinion that males
altered between the hard- and soft-shelled forms during growth. It is
now clear that the hard-shelled form, or hard stage, precedes the softshelled stages.
Investigators prior to 1959 had consistently failed to find the prehard stages of P. pisum in Mytilus edulis, and it was for this reason that
Atkins (1926) was originally of the opinion that the hard stage represents
the first crab or invasive stage. Christensen (1959), however, has been
able to correct this misinterpretation with the discovery that P. pisum
changes hosts at the hard stage and hence this is the youngest form found
in mussels. He has also found the true first crab or invasive stage of
P. pisum in Spisula solida. Although Christensen (1962, and personal
329
only slightly wider in relation to the width of the thorax than that of
the thick-shelled male. . . thin-shelled males (measure) 1-9-6.3 mm.”
Life cycle. P. pisum, like P. ostreum, includes four zoeal stages and
a single megalops. The first zoea has been described, but not very
effectively, by Thompson (1835) and later redescribed in detail by
Lebour (1928a,b). What may have been the second zoea has been
described by Lebour (19288). All of the stages, including the prezoea,
first through fourth zoeae, and megalops, have been described in detail
by Atkins (1955). The zoeal and megalopal stages are planktonic.
The most recent and complete accounts of the life cycle of P.
pisum are those given by Christensen (1959, 1962). Prior to this,
Thompson (1935) had briefly described male and female adults. He
stated that the males are hard-shelled while the females are soft-shelled.
This concept was held until Orton (1920) demonstrated the existence
of hard-shelled females which resemble males in all respects except for
their genital apertures and pleopods. Atkins (1926) has described the
molting stages of P. pisum in Mytilus edulis but followed Orton (1920)
in designating the hard-shell stage as the first crab stage (which
obviously does not correspond to the first crab stage of P. ostreum as
given by Christensen and McDermott (1958)). Later Atkins (1954,
1955) realized her error and abandoned the concept that the hard stage
is the first crab or invasive stage. Atkins (1955) has described the postembryonic development of P. pisum and still later (1958) has recorded
the occurrence of thin-shelled males which resembles stage I1 females.
Actually soft-shelled males had been found by Mercier and Poisson
(1929) but these French workers failed to realize that this is a normal
form. They were of the opinion that the soft-shelled condition had
resulted from parasitism by an entoniscid parasite. Although Atkins
(1958) has advanced our understanding of the stages in the life cycle
of P. pisum by definitely demonstrating the soft-shelled males, she
did not grasp their significance. She expressed the opinion that males
altered between the hard- and soft-shelled forms during growth. It is
now clear that the hard-shelled form, or hard stage, precedes the softshelled stages.
Investigators prior to 1959 had consistently failed to find the prehard stages of P. pisum in Mytilus edulis, and it was for this reason that
Atkins (1926) was originally of the opinion that the hard stage represents
the first crab or invasive stage. Christensen (1959), however, has been
able to correct this misinterpretation with the discovery that P. pisum
changes hosts at the hard stage and hence this is the youngest form found
in mussels. He has also found the true first crab or invasive stage of
P. pisum in Spisula solida. Although Christensen (1962, and personal