3
Fig. 2. Examples of SEM micrographs oflarval stages of Rehbachiella; (a) ventral view of stage- 10 larva (TS3) ; scale bar = 3- 0 11m); (b) lateral
view of stage-20 larva (TS8; scale bar = 100 11m) ; (c) ventral view of stage-28 larva (TSI2) with large head shield covering much of the body
(scale bar = 100 11m; (d) lateral view of stage-3D larva, head shield flexed anteriorly to expose the complete trunk (TS30; scale bar = 100 11m).
Growth and morphogenesis
Important changes during ontogenesis affect all structures of the feeding and locomotory apparatus, in particular the appendages. For example, all three naupliar
limbs are at first the same size and the mandibular coxa
is much less developed than that of the second antenna,
a typical feature of a crustacean 'ortho-nauplius'. Subsequently, the median side of the coxa grows out into a
huge grinding plate. Its inner edge differentiates into a
cutting and a grinding part. The basi pod portion of the
mandible is more important initially. Later, however,
the basipod diminishes in size and eventually its insertion area is only as large in the first stage (Fig. 4). It is
very likely that the palp was lost completely, as is the
case in all entomostracan Crustacea which complete
their morphogenesis (see below).
The postmandibular limbs appear as uniform or
bilobate buds and develop very gradually into phyllopodous limbs, but with distinct rami. Only the first
maxilla differs somewhat in starting as a small spine,
in developing only four rather than 6-8 lobate endites
at the inner edge of its protopod (actually the basipod, see below), and in a decrease in size later during
ontogeny. The protopodal endites of the limbs bear
at first only simple setae and spines. I assume that
the limbs attained functionality at a stage when they
have received three sets of differentiated enditic setae
and/or spines. The functional limbs are phyllopodous
and bear more than 200 setae along their inner edge. Yet
further growth occurs by an increase in size and addition of setae/spines. The protopods are flattened, most
likely soft-walled and backwardly curved in a C-shape
(phyllopodium). Only the outer edges are slightly more
sclerotized, but the joint-like furrows present may have
allowed some flexibility. The endopods elongate progressively during growth and become four-segmented,
without significant changes later on. The exopods also
elongate and become flat, paddle-shaped organs with
natatory setation all along the outer and distal margin.
The form and equipment of the Rehbachiella nauplius suggest that it was capable of swimming and had
the same feeding habits as free-swimming, feeding
extant nauplii. When comparing the Rehbachiella nauplius with extant (ortho)nauplii, the former exhibits an
ideal intermediate stage between those nauplii of ovoid
form and with limbs of equal length - for example of copepods, cirripeds, or even the lecithotrophic nauplii of penaeid Decapoda - and the nauplii of
Branchiopoda with their dominant second antenna and
elongate pear shape. To me, there are decidely fewer
differences than have been stated by Gauld (1959) and
whose views were adopted later by Sanders (1963). As
a matter of fact, the size of the protopod is not a distinctive character ofbranchiopod nauplii. Protopods of
similar prominence as in branchiopod nauplii are also
developed in other Crustacea, such as the lecithotroph-
Fig. 2. Examples of SEM micrographs oflarval stages of Rehbachiella; (a) ventral view of stage- 10 larva (TS3) ; scale bar = 3- 0 11m); (b) lateral
view of stage-20 larva (TS8; scale bar = 100 11m) ; (c) ventral view of stage-28 larva (TSI2) with large head shield covering much of the body
(scale bar = 100 11m; (d) lateral view of stage-3D larva, head shield flexed anteriorly to expose the complete trunk (TS30; scale bar = 100 11m).
Growth and morphogenesis
Important changes during ontogenesis affect all structures of the feeding and locomotory apparatus, in particular the appendages. For example, all three naupliar
limbs are at first the same size and the mandibular coxa
is much less developed than that of the second antenna,
a typical feature of a crustacean 'ortho-nauplius'. Subsequently, the median side of the coxa grows out into a
huge grinding plate. Its inner edge differentiates into a
cutting and a grinding part. The basi pod portion of the
mandible is more important initially. Later, however,
the basipod diminishes in size and eventually its insertion area is only as large in the first stage (Fig. 4). It is
very likely that the palp was lost completely, as is the
case in all entomostracan Crustacea which complete
their morphogenesis (see below).
The postmandibular limbs appear as uniform or
bilobate buds and develop very gradually into phyllopodous limbs, but with distinct rami. Only the first
maxilla differs somewhat in starting as a small spine,
in developing only four rather than 6-8 lobate endites
at the inner edge of its protopod (actually the basipod, see below), and in a decrease in size later during
ontogeny. The protopodal endites of the limbs bear
at first only simple setae and spines. I assume that
the limbs attained functionality at a stage when they
have received three sets of differentiated enditic setae
and/or spines. The functional limbs are phyllopodous
and bear more than 200 setae along their inner edge. Yet
further growth occurs by an increase in size and addition of setae/spines. The protopods are flattened, most
likely soft-walled and backwardly curved in a C-shape
(phyllopodium). Only the outer edges are slightly more
sclerotized, but the joint-like furrows present may have
allowed some flexibility. The endopods elongate progressively during growth and become four-segmented,
without significant changes later on. The exopods also
elongate and become flat, paddle-shaped organs with
natatory setation all along the outer and distal margin.
The form and equipment of the Rehbachiella nauplius suggest that it was capable of swimming and had
the same feeding habits as free-swimming, feeding
extant nauplii. When comparing the Rehbachiella nauplius with extant (ortho)nauplii, the former exhibits an
ideal intermediate stage between those nauplii of ovoid
form and with limbs of equal length - for example of copepods, cirripeds, or even the lecithotrophic nauplii of penaeid Decapoda - and the nauplii of
Branchiopoda with their dominant second antenna and
elongate pear shape. To me, there are decidely fewer
differences than have been stated by Gauld (1959) and
whose views were adopted later by Sanders (1963). As
a matter of fact, the size of the protopod is not a distinctive character ofbranchiopod nauplii. Protopods of
similar prominence as in branchiopod nauplii are also
developed in other Crustacea, such as the lecithotroph-