Terrestrial and freshwater gastropods have simpli-fied their development, and their embryonic and larval stages are fixed on egg capsules or the female body direct development.. The Gast
Trang 1larva, and a similar larval type is developed in all
molluscan groups The trochophore larvae may be
free swimming, as in the ancient gastropod groups
(Patellogastropoda and Archaeogastropoda), or may
occur in egg capsules, as in more advanced
gastro-pods The last larval stage is termed veliger, which
typically bears two ciliate paddles (velum), sometimes
subdivided into several lobes If free-swimming
gas-tropod larvae use planktic organisms for their
nutri-tion, their development is termed planktotrophic
Marine gastropods with such development have
small eggs, but numbering over half a million
Plank-totrophic larvae may stay planktic for several months
and thus can be carried for long distances by oceanic
currents The gastropods, however, developed
an-other ontogenetic strategy in which their larvae were
not dependent on an external food source, but on
the yolk of their eggs Gastropods with such a
non-planktotrophic development (lecithotrophic)
typic-ally produce fewer eggs, which are relatively large
The larval stages end with a metamorphosis that
involves anatomical and physiological reorganization
of the larval body into the juvenile, post-larval body
Terrestrial and freshwater gastropods have
simpli-fied their development, and their embryonic and
larval stages are fixed on egg capsules or the female
body (direct development) Such ontogenetic changes
considerably decreased their dispersal potential
The Gastropod Shell
Gastropods are not only one of the most diverse
animal groups, but the morphology of their shells is
extremely varied (Figures 1 and 2) During more than
500 million years of evolution, they developed shells
with various shapes and ornament, ranging in size
from about 1 mm up to more than 1 m (Eocene
Campaniloidea, Caenogastropoda) The shell and its
ornament may be broadly linked to the mode of
gastropod life (e.g., origin of limpet-shaped shells in
unrelated gastropod groups) Generally, the most
ornate shells occur in tropical marine environments,
but freshwater and terrestrial gastropods are often
less ornate
Protoconch and Teleoconch
In shell-bearing gastropods, the shell grows during
almost the whole of their ontogeny The part of the
shell formed during the embryonic and larval stages is
called a protoconch (Figure 3), and that growing after
metamorphosis is termed a teleoconch The main
gas-tropod groups differ in their early development, which
is reflected in their protoconch morphology The
more ancient gastropod groups (Patellogastropoda
and Archaeogastropoda) have the simplest shell ontogeny and their protoconchs have only an embry-onic shell (protoconch I), which is followed by a teleoconch (Figures 3B and 3F–3H) On the other hand, the protoconchs of more advanced gastropods (Neritimorpha, Caenogastropoda, and Heterobran-chia) consist of an embryonic shell (protoconch I) and a subsequent larval shell (protoconch II) In most caenogastropods, the larval shells have different ornament from the teleoconchs (Figures 3K and 3L), and both shells are coiled in the same direction (such a condition is termed homeostrophic; Figure 4) In contrast, in the Heterobranchia with planktotrophic development, the protoconchs are coiled in the op-posite direction to the teleoconchs (Figures 3J and 4) Such shells are termed heterostrophic The Neritimor-pha form typical, strongly convolute protoconchs during planktotrophic development, which are homeostrophic (Figures 3A, 3N, and 4) Higher gas-tropods with non-planktotrophic development (some marine, freshwater, and terrestrial gastropods) have simplified their early ontogeny and thus also the morphology of their protoconchs The latter strategy
is documented from the Devonian (400 Ma)
Operculum
The majority of gastropods have a lid-like structure (operculum) to close their aperture This operculum is present in all living gastropods during their larval stages, but is lost in some adults (e.g., limpets and the majority of terrestrial gastropods) The opercu-lum is mostly horny (corneous) and may be tightly (multispiral) or loosely (paucispiral) coiled or concen-tric Some gastropod groups have calcareous oper-cula, and the oldest operculum known is from the Ordovician (Macluritoidea)
Shell Structure
Most gastropod shells are composed of an outer or-ganic layer (periostracum) and an inner, mostly much thicker, calcified layer The colour pattern typical of many gastropod shells (Figure 1) is formed by differ-ent organic pigmdiffer-ents which are limited to the perios-tracum and the uppermost calcified layer This shell feature, sometimes reflecting the mode of life, has been known since the Palaeozoic (Figure 5) The inner layers of gastropod shells consist of minute calcium carbonate crystals (aragonite or calcite) in
an organic matrix There are over 20 structural types of gastropod shell and, in general, more ancient groups exhibit more diverse shell structures The Patellogastropoda (Eogastropoda) had the most com-plex shell structure On the other hand, the majority
of the higher gastropods have developed simple
380 FOSSIL INVERTEBRATES/Gastropods