Molluscs Flashcards
Single, undivided, coiled shell or shell-less. Mantle cavity faces the front. Muscular foot is flattened and used for locomotion. Head is well developed with eyes and tentacles, e.g., snails and slugs. Late Cambrian-Recent
Class Gastropoda
Two-valved, hinged shell encloses the body. No head but foot well developed. Gills are modified for respiration and filter-feeding, e.g., mussels, oysters, and scallops. Lower Cambrian-Recent
Class Bivalvia
Internal or external shell or shell-less. Head and sensory organs well developed. Living representatives are intelligent and actively
carnivorous. Late Cambrian-Recent
Class Cephalopoda
External, buoyant, chambered shell. Chambers are connected and partition walls are flat, e.g., Nautilus. Upper Cambrian-Recent
Subclass Nautiloidea
External, buoyant, chambered shell. Chambers are connected and partition walls are folded, e.g., ammonites. Lower Devonian-Upper
Cretaceous
Subclass Ammonoidea
Shell internal and reduced or absent, e.g., belemnites, squids, and octopuses. Carboniferous-Recent
Subclass Coleoidea
have an elongate, unsegmented body with a distinct head. The internal organs are held between a muscular foot, a modified lower part of the body, and a calcareous shell secreted by an underlying tissue known as the mantle
Molluscs
are the largest and most diverse class of molluscs. They live in marine, freshwater, and terrestrial environments and have exploited the widest variety of habitats and developed a remarkable range of feeding strategies may have a calcareous shell or be entirely soft
bodied.
Gastropods
Gill in front, Cap shaped or conispiral, mostly marine, ex: limpets, winkles, whelks
Main subclass gastropods:
Prosobranchiata
Gill behind, due to distortion; shell lost or very reduced; marine; Ex:Sea-slugs and sea hares
Opisthobranchiata
Mantle cavity modified into lung; where present, conspiral or planispiral; terrestrial; Ex: Land snail and slugs
Pulmonata
Most fossil gastropods are prosobranchs and this subclass is divided into three orders:
- Archaeogastropoda:
Cambrian to
Recent - Mesogastropoda:
Carboniferous to
Recent - Neogastropoda:
Cretaceous to Recent
are laterally compressed molluscs enclosed within a pair of hinged shells or valves. Valves are closed by the adductor muscles. The shell is opened by relaxing these muscles and
water currents are drawn into the cavity
Bivalves
Types of bivalves:
Numerous teeth in radial or subparallel arrangement
Taxodont
Types of bivalves:
Small simple teeth at valve margin
Dysodont
Very large teeth positioned either side of ligament pit
Isodont
Types of bivalves:
Large grooved teeth
Schizodont
Types of bivalves:
Large teeth(termed cardinal teeth) flanked by smaller (lateral) teeth
Heterodont
Types of bivalves:
Teeth reduced to ridges or absent
Desmodont
The major life habits of bivalves are:
(i) burrowing in soft substrates;
(ii) boring and cavity dwelling;
(iii) attached (cemented or by byssus threads);
(iv) unattached recumbant; and
(v) intermittant swimming
Bivalves that burrow in soft substrates tend to be equivalved and have a distinct pallial sinus
Bivalves that bore into hard
substrates typically have
elongate thin shells that are
resistant to abrasion.
is acheved by the foot, which penetrates the sediment and swells. The muscles in the foot then contract, drawing the shell down through the sediment.
Burrowing
Epifaunal bivalves exploit three living
strategies:
1.) Attachment to the substrate by byssus threads
2.) Cementation to hard surfaces
3.) Recumbent, free lying on the sediment
surface stabilized by the shell morphology
Bivalves that are able to swim do so only
intermittently. Shells tend to be thin, to reduce weight. There is only one large adductor muscle scar which provides the strong contractions needed for swimming.
Swimming bivalves
are the most morphologically complex group of molluscs. They occupy the same ecological niche as fish and they are arguably the most sophisticated group of invertebrates. The class includes active, jet-propelled predators with highly developed sensory structures. All cephalopods are marine.
Cephalopods
Cephalopods are divided into three
subclasses:
Nautiloidea, Ammonoidea, and
Coleoidea.
have an external, chambered
shell with simple sutures between its chambers
Nautiloids
also have an external shell, always coiled, with variable and more complicated sutures.
Ammonoids
have an internal and reduced shell. In some coleoids the shell is absent
Coleoids