\0
Weisz (1946, 1947) has proposed that comparison
of ontogenetic series should refer to developmental
status, i.e. by monitoring the addition of segments.
This method provides an absolute scale and is independent of individual and environmen- tal modifications.
It is still appropriate when a sequence is incompletely
known, abbreviated, or if ontogeny is epimeric.
Focusing on progressive development of segments
and structures, the method advanced by Weisz has
readily revealed basic patterns for each of the major
groups, from which further modifications within the
groups can be elucidated. It has also shown that it is
important not to mismatch a true ortho-nauplius with
later hatching stages. This is most evident if, due to
delay of limb development, larvae develop no more
than the naupliar three pairs of limbs during a series of
moults, such as in rhizocephalan Cirripedia. Correlating ontogenetic series by segment addition permits the
interpretation of sequence patterns by the number of
moults in relation to particular segments stages and the
status of development of limbs at a gi ven stage. In other words, 'jumps' in development between two moults
become apparent on account of the gap between them,
indicating the number of stages is condensed. Nevertheless this method also relies on a precise documentation of the sequence and marker points that permit
correlation.
Table 1, drawn according to the method of Weisz
(modified Tab. 4 ofWalossek 1993) shows the developmental sequences of the major crustacean groups.
Despite all variability there seems to be a ground pattern for Crustacea, which is recognizable in much of the
strictly anameric series of Rehbaehiella and Artemia as
an excellent example of gradual growth in extant Crustacea. After a phase, in which the segments ofthe maxillae appear successively (4 stages in Rehbaehiella),
all thoracic structures are developed gradually (from
TS 1 ion; 26 stages in Rehbaehiella). Limbs appear
with a delay of one to a few stages relative to the segments and develop gradually from the anterior towards
the budding zone in front of the telson. In the postlarval phase reaching to the reproductive stage (not
documented in Rehbaehiella), further differentiation
occurs, such as segmentation of the abdomen and modification or reduction of naupliar structures (example:
Artemia; note that there are actually 13 thoracomeres in
Anostraca according to Benesch 1969, who counts the
genital segments as thoracic). Additional moults may
occur also after the final number of body segments has
been achieved. Artemia shows a slight abbreviation
of the series only in the earliest phase (2 instead of
four), while the remaining series is strictly anameric,
reaching 50% of Rehbaehiella stages. Formation of
each trunk segment occurs in one step in Artemia, but
according to Weisz (1946) a two-phased development
of segments, such as in Rehbaehiella, is indicated.
Maxillopoda deviate from this pattern mainly in terminating development after having budded off \0 trunk
segments (11 in the ground pattern, as revealed by
the Upper Cambrian Skaracarida; Muller & Walossek
1985). Copepoda still exhibit much of a plesiomorphic
anameric sequence, save for a short 'jump' between the
sixth nauplius and the first copepodid, while ontogeny of Mystacocarida shows adaptation to life in the
interstitial environment (e.g., limb reduction). The
sequence of Cirripedia as an example of thecostracan growth, is split into two phases. The 'naupliar' phase is characterized by a suppression of trunk
limb development and lack of external trunk segmentation. At the moult to the 'cypris' phase all trunk
limbs appear simultaneuosly, and the trunk is subdivided into a segmented thorax and abdomen. Since the
'nauplius' already bears all thoracopods underneath the
cuticle (cf. Nilsson-Cantell 1921: Fig. 5a for Sealpellum gibberum), there is no direct correlation between
the copepodids of Copepoda and thecocastran cyprids
(or their equivalents). A term like 'podid' for all maxillopod larvae of the second phase, as proposed by
Newman (1983) camouflages the striking differences
between the two maxillopod lines. Again, the six nauplii of Copepoda are not developmental equivalents of
the maximally six 'nauplii' of the first phase of the
Thecocastra, which becomes readily apparent when
correlating the stages in Table 1.
Eumalacostraca, whether or not starting their
postembryonic growth with a nauplius stage, deviate from the anameric sequence in having developed
several distinct phases in which typically sets of segments and limbs appear simultaneously. No limbless
abdominal segments appear throughout ontogeny, and
developmental 'jumps' may not even conform with the
phases recognized by man.
Development of the Cephalocarida has been
described as very anameric and claimed to be the
most gradual type among the Crustacea (Sanders
1963). However, Fryer (1983) has pointed out already
that their development is only superficially a gradual process. Hutehinsoniella hatches with larva having already two trunk segments (TS2), and hence has
no nauplius stage, while Lightiella starts even with
a TS7 stage. It takes just one more moult for Lightiella in achieve the TS 13 stage. Besides 'jumps' in
Weisz (1946, 1947) has proposed that comparison
of ontogenetic series should refer to developmental
status, i.e. by monitoring the addition of segments.
This method provides an absolute scale and is independent of individual and environmen- tal modifications.
It is still appropriate when a sequence is incompletely
known, abbreviated, or if ontogeny is epimeric.
Focusing on progressive development of segments
and structures, the method advanced by Weisz has
readily revealed basic patterns for each of the major
groups, from which further modifications within the
groups can be elucidated. It has also shown that it is
important not to mismatch a true ortho-nauplius with
later hatching stages. This is most evident if, due to
delay of limb development, larvae develop no more
than the naupliar three pairs of limbs during a series of
moults, such as in rhizocephalan Cirripedia. Correlating ontogenetic series by segment addition permits the
interpretation of sequence patterns by the number of
moults in relation to particular segments stages and the
status of development of limbs at a gi ven stage. In other words, 'jumps' in development between two moults
become apparent on account of the gap between them,
indicating the number of stages is condensed. Nevertheless this method also relies on a precise documentation of the sequence and marker points that permit
correlation.
Table 1, drawn according to the method of Weisz
(modified Tab. 4 ofWalossek 1993) shows the developmental sequences of the major crustacean groups.
Despite all variability there seems to be a ground pattern for Crustacea, which is recognizable in much of the
strictly anameric series of Rehbaehiella and Artemia as
an excellent example of gradual growth in extant Crustacea. After a phase, in which the segments ofthe maxillae appear successively (4 stages in Rehbaehiella),
all thoracic structures are developed gradually (from
TS 1 ion; 26 stages in Rehbaehiella). Limbs appear
with a delay of one to a few stages relative to the segments and develop gradually from the anterior towards
the budding zone in front of the telson. In the postlarval phase reaching to the reproductive stage (not
documented in Rehbaehiella), further differentiation
occurs, such as segmentation of the abdomen and modification or reduction of naupliar structures (example:
Artemia; note that there are actually 13 thoracomeres in
Anostraca according to Benesch 1969, who counts the
genital segments as thoracic). Additional moults may
occur also after the final number of body segments has
been achieved. Artemia shows a slight abbreviation
of the series only in the earliest phase (2 instead of
four), while the remaining series is strictly anameric,
reaching 50% of Rehbaehiella stages. Formation of
each trunk segment occurs in one step in Artemia, but
according to Weisz (1946) a two-phased development
of segments, such as in Rehbaehiella, is indicated.
Maxillopoda deviate from this pattern mainly in terminating development after having budded off \0 trunk
segments (11 in the ground pattern, as revealed by
the Upper Cambrian Skaracarida; Muller & Walossek
1985). Copepoda still exhibit much of a plesiomorphic
anameric sequence, save for a short 'jump' between the
sixth nauplius and the first copepodid, while ontogeny of Mystacocarida shows adaptation to life in the
interstitial environment (e.g., limb reduction). The
sequence of Cirripedia as an example of thecostracan growth, is split into two phases. The 'naupliar' phase is characterized by a suppression of trunk
limb development and lack of external trunk segmentation. At the moult to the 'cypris' phase all trunk
limbs appear simultaneuosly, and the trunk is subdivided into a segmented thorax and abdomen. Since the
'nauplius' already bears all thoracopods underneath the
cuticle (cf. Nilsson-Cantell 1921: Fig. 5a for Sealpellum gibberum), there is no direct correlation between
the copepodids of Copepoda and thecocastran cyprids
(or their equivalents). A term like 'podid' for all maxillopod larvae of the second phase, as proposed by
Newman (1983) camouflages the striking differences
between the two maxillopod lines. Again, the six nauplii of Copepoda are not developmental equivalents of
the maximally six 'nauplii' of the first phase of the
Thecocastra, which becomes readily apparent when
correlating the stages in Table 1.
Eumalacostraca, whether or not starting their
postembryonic growth with a nauplius stage, deviate from the anameric sequence in having developed
several distinct phases in which typically sets of segments and limbs appear simultaneously. No limbless
abdominal segments appear throughout ontogeny, and
developmental 'jumps' may not even conform with the
phases recognized by man.
Development of the Cephalocarida has been
described as very anameric and claimed to be the
most gradual type among the Crustacea (Sanders
1963). However, Fryer (1983) has pointed out already
that their development is only superficially a gradual process. Hutehinsoniella hatches with larva having already two trunk segments (TS2), and hence has
no nauplius stage, while Lightiella starts even with
a TS7 stage. It takes just one more moult for Lightiella in achieve the TS 13 stage. Besides 'jumps' in