Palæos:		Mollusca
METAZOA		Mollusca

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Phylum Mollusca

PROTOSTOMIA |--ARTHROPODA | |--Trilobita | `--PANCRUSTACEA | |--CRUSTACEA | `--INSECTA `--LOPHOTROCHOZOA |--MOLLUSCA | BASAL MOLLUSCA | |--Diasoma | | |--Rostroconchia | | `--BIVALVIA | | |--Protobranchia | | `--HETERODONTA | `--CYRTOSOMA | |--CEPHALOPODA | `--GASTROPODA | |--Eogastropoda | `--APOGASTROPODA | |--Caenogastropoda | `--HETEROBRANCHIA | |--Opisthobranchia | `--PULMONATA `--ANNELIDA

Unit Contents Mollusca  Shell Morphology Basal Mollusca   Bivalvia Cyrtosoma Gastropoda Pulmonata   Mollusk Phylogeny Glossary Cladogram References

Page Contents Mollusca   Physiology Diversity   Habitat   Taxonomic History   Evolutionary History   Systematics   Links

The Mollusks are a large and diverse group of soft-bodied unsegmented animals.  Nearly 130,000 recent species are known, and some 35,000 fossil species.  They include many familiar animals, like snails, clams, squid, octopi, etc, as well as others not so well known.  They range in size from microscopic forms to the giant squid (Architeuthis), and have a long and venerable history appearing during earliest Cambrian time, if not before.

Physiology

All mollusks possess some or all of the following characteristics:

• a muscular fleshy foot which with gastropods (snails) is used to crawl along with but in cephalopods is modified into tentacles
• a visceral mass containing the digestive, excretory, and reproductive organs
• a mantle, usually two folds that enclose the gills or lungs, and also secretes the protective shell
• a radula, a tongue-like feeding organ, essentially a sort of built-in saw equipped with rows of microscopic teeth that is incredibly effective at scraping food off hard surfaces or grinding through any substance (some carnivorous snails are able to bore a whole through the shells of other mollusks)
• a special respiratory gill (the ctenidium)
• a shell made of calcium carbonate. This fossilizes easily, which explains the rich fossils record the mollusks possess.

Although not all mollusks share all these characteristics, generally all mollusks have at least some of them.

Molluscan Diversity

Despite sharing these common features, the mollusks are an incredibly diverse group. The following diagram shows the major classes, which as can be seen are all based on variation in this same body plan.

This diagram presents a stylized relationship between the different Molluscan groups, with all evolving from a "hypothetical ancestral mollusk". It is now known that molluscan phylogeny is a lot more complex than this.

Habitat

As one of the most successful forms of animal life, the mollusks have conquered almost every habitat and exist in all the oceans (from shallow tidal pools to the deepest trenches), in fresh water, and on land.  The only environment they cannot cope with are very dry regions, as their moist skin is easily desiccated. Even so, the greatest diversity of mollusks species is to be found in marine environments

page from Volume 2 of the three volume set The Science of Life by H. G. Wells, Julian Huxley and G. P. Wells (1929-39).

Taxonomic History

When Linnaeus was formulating his system of binomial nomenclature the mollusks were grouped (along with most invertebrates) under vermes, worms.  The name Mollusca (from the Latin mollis "soft"), was first used by the great French zoologist Cuvier in 1798 to refer to cephalopods like squids and cuttlefish.  It was later extended to include other organisms of this group, such as snails and bivalves.

Evolutionary History

The mollusks first appeared during the earliest Cambrian (Tommotian), at the very start of radiation of coelomate animals.  The 35,000 fossil species that are known are without doubt only a small fraction of those that have ever lived.  As the Cambrian period progressed, many types appeared that soon died out.  Although previously included among the conventional classes of mollusks it is increasingly argued that these represent distinct classes, early experiments in molluscan evolution, or transition forms.  However, the exact number of extinct classes remains debatable

By the Ordovician period the three main classes - Gastropoda, Bivalvia, and Cephalopoda - had increased greatly in number and diversity.  This was a period of major evolutionary diversity for the phylum, and they became common in most marine ecosystems

The Devonian saw bivalves invade freshwater, and the first land snails appeared during the Carboniferous. During the Devonian also, the ammoniod took over from the nautiloids as the dominant cephalopod group, and these creatures continued to flourish throughout the entire Mesozoic, living alongside species of bivalves and gastropods not very different to modern forms. (Oysters for example were common during the Jurassic and Cretaceous, while scallops date back to the Paleozoic). By mid Cretaceous most mollusks, like most invertebrates and fish, were essentially like modern forms, except for the Cephalopods which were still represented largely by the Mesozoic ammonites and belemnites.  The ammonites were decimated by the K-T extinction event, as were the reef-building rudist bivalves (although these were on the decline for some time), but the coleoids (soft-bodied squids, octopi etc) continued to evolve quite happily.

Systematics

Traditionally seven classes are recognized: Aplacophora (primitive worm-like shell-less forms), Polyplacophora (chitons, these were at one tile included with the Aplacophora), Monoplacophora (primitive ancestral forms), Gastropoda (snails and slugs), Bivalvia (clams, mussels, oysters, etc), Scaphopoda (tooth and tusk shells), and Cephalopoda (squids, octopus, nautilus, cuttlefish), of which only Gastropoda, Bivalvia, and Cephalopoda have large numbers of species and a strong fossil record.  However it is becoming increasingly recognized that there were more groups that have since died out.

With the falling out of favor of Linnean methodology, recent work in molluscan systematics is done is done via cladistic analysis. The following is a recent cladogram showing molluscan relationships.

My own position, oft-stated, is that the Linnean and cladistic methodologies are complementary rather than contradictory. Cladistics provides a means of drawing the family tree of life, while the Linnean hierarchy enables us to construct a useful, if arbitrary, arrangement for cataloging and comparative purposes. The following table therefore represents a possible list of molluscan classes (including extinct forms) are known or have been proposed (note - not all experts disagree as to the status of the various Paleozoic minor classes!!!).

	Solenogastres (= Neomeniomorpha; Aplacophora in part)	no fossil record	very small creeping worm-like soft bodied shell-less forms
	Caudofoveata (= Chaetodermomorpha; Aplacophora in part)	?Silurian to Recent	very small deep sea worm-like soft bodied burrowing shell-less forms. The Silurian Acaenoplax, which resembles both Caudofoveata and Polyplacophora, may be an early form of this lineage.
	Polyplacophora (= Amphineura in part)	Furongian to Recent	The chitons, which have eight plates or "valves" which are overlapping in post-Mesozoic forms. The foot is broad; the head reduced. The soft parts show some segmentation. The animal generally lives attached to rocks in the inter-tidal zone. Appear to be distantly related to the Solenogastres and Caudofoveata (as ancestors/cousins), and the Tryblidiida
	Tryblidiida (= Monoplacophora sensu stricto = Tryblidiida = Tergomya in part)	Furongian to Recent	Creeping cap-shelled forms with paired muscle attachments. The most primitive conchiferan group. A few dozen deep water recent species remain as "living fossils". As with the Polyplacophora the soft parts show some segmentation. Probably evolved from a soft-bodied ancestor, possibly Polyplacophora relationships.
	Stenothecoida (= Monoplacophora in part)	Early to Middle Cambrian	Small group of little known bivalved forms. There relationship with other molluscan groups is unclear, and it is not agreed whether they qualify as class status. No descendents, ancestors unknown.
	Helcionelloida (= Monoplacophora in part; paraphyletic)	Earliest to Late Cambrian	an ancestral lineage or grade. Consist of creeping or weakly infaunal forms with cap-shaped or spiral shell. Probably evolved from a soft-bodied ancestor, as the direction of shell coiling is opposite that of the Tryblidiida, which they predate in any case. Generally considered directly or indirectly the ancestors of all conchiferan mollusks other than the Tryblidiida.
	Bivalvia (= Pelycopoda)	Early Cambrian to Recent	mostly sedentary or burrowing forms with a shell of two valves hinged dorsally. The foot is generally hatchet-shaped (hence the name "Pelycopod"); the head lacking; a very diverse group, includes epifaunal or infaunal forms, mostly marine but some freshwater species. Clams oysters, mussels, etc. Generally considered to have evolved from rostroconches, although this would relate them to helcionelloids, if not actually Scaphopods and Cephalopods, and the weight of current molecular and cladistic evidence indicates the bivalves diverged from the main Conchiferan lineage quite early.
	Paragastropoda	Early Cambrian to Devonian	Small group of superficially snail-like forms, and perhaps snail-like habits, distinguished by lack of torsion. Evolved from helcionelloids, no descendents. It is not even certain if this is a natural or a polyphyletic group.
	"Tergomya" in part (= Monoplacophora in part; polyphyletic)	Furongian to Devonian	A probably artificial assemblage of creeping cap- or partly spiral shelled forms with single or paired muscle attachments. The Tryblidiida are usually included here but they are certainly a more basal group. Non-tryblidiidan Tergomya may have evolved probably several times from helcionelloids, although some may be related to the Tryblidiida. Some of the helcionelloid-descended forms would seem to be related to the Gastropods
	Gastropoda	Furongian to Recent	Diverse group of shelled or shell-less forms, evolved from a spiral-shelled helcionelloid or helcionelloid-descended "tergomyan" ancestor that underwent torsion. The body is usually asymmetrical, with a distinct head, pair of eyes, and one or two pairs of tentacles. The shell is usually spirally-coiled, but may (in the case of limpets) be cap shaped, and in some species is vestigial or completely absent. The foot is broad and used to creep along the substrate. Includes marine, freshwater, and terrestrial species (the only mollusks to invade the land). Snails, slugs and their relatives
	Rostroconchia	Early Cambrian to Late Permian	A Paleozoic group of infaunal forms. The shell consists of two valves joined in adults by an enclosed hinge. Evolved from helcionelloids. The posterior of the shell is usually produced into an elongate tube, presumably for inhalant and exhalent siphons. Originally considered to have given rise to both Scaphopods and Bivalves, as they share many similarities with both those groups. But relationships among early molluscan classes remain controversial
	Scaphopoda	Devonian to Recent	small semi-infaunal forms, the tubular shell open at both ends and resembles a miniature elephant's tusk, with one end larger than the other. The foot is conical; there are no gills. Probably evolved from rostroconches. Shown to be related to the Cephalopods.
	Cricoconarida (= Tentaculitoidea)	Middle Cambrian to Devonian	very small pelagic forms with tubular shells. They are thought to be related to the Cephalopods, and seem to have born a number of arms.
	Cephalopoda	Latest Cambrian to Recent	diverse group of large intelligent predaceous forms, with a large external shell (most Paleozoic and Mesozoic forms, and current Nautilus), a vestigial internal shell, or (in the case of the octopus) the shell is lost altogether. The head is large with well-developed eyes, and armed with horny chitinous jaws and many arms or tentacles for grasping prey; the head fused to the foot (hence the name). Evolved from helcionelloids.
	Hyolitha	Earliest Cambrian to Permian	Enigmatic animals with small conical shells, shell-microstructure indicates close to mollusks but experts do not agree whether they actually were mollusks.

Links

Links

Phylum Mollusca - a good brief introduction to the Mollusks

The Living World of Molluscs - another very good intro, a bit more detailed

Mollusca

Malacology - from the Australian Museum On-line

Mollusca - a good introduction, from Biomedia (Glasgow University Zoology Museum)

Index of Mollusks by Ralph Body - mostly still under construction, although the Aplacophora are well covered

The Classes of Phylum Mollusca - short overview of each of the extant classes

Systema Naturae 2000 / Classification - Phylum Mollusca

LAB 3 Chapter 8: Phylum Molusca - some lecture notes and images.