Mollusca

Nautilus

The molluscs ( Mollusca ) or mollusks (Latin molluscus " soft " ) form a species- rich and form- phylum within the tissues Animals ( Eumetazoa ) and join in the sea, on land and in fresh water before. The science of the molluscs is called ( " soft " Greek malakos ) as Malacology or Malakozoologie.

• 8.1 Outer systematics
• 8.2 Internal systematics

Anatomy

The body of the molluscs is, except for the Contracts Manager and approaches of chitons, unsegmented (pseudo segmentation ), built originally bilaterally symmetrical and consists of four sections: the front head, muscular foot, a visceral sac and a jacket. The shell separates in many species a exoskeleton, the shell from. This bowl can, for example in the form of two flaps, as in the shells or spiral wound as in the screw housing occur. The space between the body surface and mantle is called cladding or air pocket.

The skin of the molluscs is single-layered, soft, gland- rich and often provided with lashes.

Most molluscs have shells made of calcium carbonate and proteins, which serve as protection and - form a kind of exoskeleton - limited in scope. This mollusc has in all groups of unitary construction made ​​of three layers:

• Periostracum ( Schalenhäutchen from the glycoprotein Conchin )
• Ostracum (outer prismatic layer of vertical limestone prisms)
• Hypostracum (inner nacre layer of lime - aragonite )

It is absent in the Wurmmollusken and there has been replaced by calcareous spicules in the skin. In some cephalopods in the body is known as a cuttlefish of lime or horn, which acts as an internal skeleton. This cuttle-bone, called for squid as Gladius, represents the inward grown, chambered shell of the ancestral species of cephalopods; they do is still to be found in the Nautilus species.

Nutrition

The digestive system of molluscs differs considerably according to their physiological needs. Particularly stable biopolymers are often found on the food taken and crushed structures ( radula or parrot beak -like jaws of cephalopods ) in the form of chitin. Individual herbivore molluscs also use endogenous cellulases to degrade cellulose, which has been demonstrated in some snails such as the snail and some shells in their digestive system: Corbicula japonica and pedicellatus Lyrodus. The origin of the cellulase is fed back to the last common ancestor of the Bilateria.

Sense organs

Sensory cells are found among other things in the form of tactile, olfactory, gustatory and photoreceptor cells.

While simple photoreceptor cells in most groups occur, cephalopods, snails and mussels some real eyes have trained. The sense organs are particularly innervated by the cerebral ganglion. Your highest level of organization they reach in the eyes of cephalopods, which are similar in their performance and their construction through the eyes of vertebrates, but perform differently.

Other senses are often existing tentacles. They serve some of the mechanoreception ( touch), chemoreception (taste ), olfaction ( smell ), photoreception ( light perception ) as well as the perception of flow. Other characteristic sense organs are the osphradia that lie in aquatic forms the mantle edge, projecting into the mantle cavity and serve the chemical sensory perception.

Locomotion and Sessilität

Locomotion

Snails: Snails move initially before crawling. Typical land snails pull the rear end of her foot at regular, short intervals to the front and put it on there again, resulting in wavy lines form at the bottom of the foot.

Mussels: mussels Typical slow to change their position. You are looking for a suitable place to dig with their muscular foot or baste. By muscle contraction in the foot, they can change their position. Some mussels ( mussel, zebra mussel ) in addition solve byssus threads from the Fußrinne with which they can hold onto firm ground or other mussels and change their position.

Cephalopods: Cuttlefish have two techniques for locomotion.

• By wave hitting the Flossensaums. However, this is only a relatively slow locomotion possible.
• If it swirls breathing water into the mantle cavity, then close and the water discharge quickly through the funnel, by contracting the muscular mantle wall, they move on quickly. This process is called recoil principle.

Attachment, protection

Many molluscs produce silk and silk proteins:

Shells, such as mussels, produce byssus ( fine linen ) to attach themselves, often only in the juvenile stage. Fine linen is the silk of insects homologous in structure, it contains the silk fibroin and sericin proteins.

Many molluscs of different classes of shell molluscs including some cephalopods (such as nautilus ) form shells, the innermost layer, the hypostracum, consists essentially of nacre, which contains silk proteins.

The first plants to silk production are to be sought in the common ancestor of molluscs and arthropods. During further evolution resulted from heterogeneity of solutions that is not yet completely understood.

Reproduction and Development

Most shells, all cephalopods and many snails are dioecious. Hermaphroditism is however common in some groups or even the norm, especially in the lung worm. Fertilization takes place initially in the open water in which the eggs and sperm are released; many derived Molluskengruppen to internal fertilization has developed, for example, in all living on the mainland snails.

The development of the molluscs is usually done by a Spiralfurchung. In the further course first develops a larval stage. This can be a Hüllglockenlarve, a Präveliger or Veliger be externally resemble a trochophore larva. The yolk-rich eggs of cephalopods exhibit a discoidal cleavage and show a direct development. In some groups, particularly in the living in fresh water and on land snails, the development is also directly and freely without living larval stage.

Some species operate parental care: Types of Contracts Manager ( Monoplacophora ) keep the embryos to hatch in the mantle cavity, similar to the pea clams ( Pisidiidae ) make in freshwater. Octopuses of the genus Octopus guarding her nest and scattered tropical freshwater snails show forms of parental care.

Distribution and number of species

Molluscs live with the exception of the polar regions and high mountains in all habitats on earth. However, most mollusks live in the sea, almost all groups are exclusively marine ( cephalopods, Kahnfüßer, Contracts Manager, Wurmmollusken, chitons ). The mussels are also found in fresh water and in some cases even in moist soil, such as the pea clams ( Pisidium spp.). All other habitats are populated by the largest group of molluscs, the snails.

The total number of species is given very different, sometimes running at well over 100,000 species to sometimes only 50-60,000 species. Main reasons for the different specifications are unclear Artabgrenzungen and in particular the lack of a general overview of the biodiversity of the various sub- groups.

Paleontology and early evolution

Fossil molluscs occur with certainty since the early Cambrian, but the phylum probably originated already in the late Precambrian. In many cases, it is assumed that the first mollusks arose before about 650 million years ago from small worm-like ancestors. These were perhaps only 1-3 mm in size, unsegmented and equipped with a chitinous cuticle with embedded Aragonitschuppen on the top and a ciliated sliding sole on the bottom.

System

The evolution of the molluscan classes and their phylogenetic relationship to each other is not yet fully understood.

Outer systematics

Together with other animal strains, the molluscs are now placed in the root group of lophotrochozoans. Within this group are currently considered the calyx worms ( Entoprocta, also called Kamptozoa ) as the closest relatives. The former often voiced suspicion of a closer relationship of the molluscs to annelids ( Annelida ) is rarely discussed.

As a central and systematically significant characteristic of the tribe of the molluscs in the sense of autapomorphy (shared derived character ) is considered the radula ( rasping tongue ). The other key morphological features in the original blueprint of the molluscs are the threefold body into head, foot and visceral sac, the formation of a sheath having a sheath channel, the reduction of the coelom to the Perikardialsystem as well as the formation of an (originally always ) open vascular system with a ventricle and one or more arteries. In addition, the shell is a particular important feature of almost all molluscs, but with the primary one-piece shell with a typical three-layer structure only in the shell mollusks ( Conchifera ) occurs.

Inside systematics

To the family of eight living molluscs and several extinct classes are counted. The following three classes are not typical mollusc shells. They are referred to as a spiked molluscs ( Aculifera ). Molecular biological studies suggest that this group actually emerged from a common ancestor.

• Schildfüßer ( Caudofoveata ), about 70 species, Sea (Deep Sea )
• Furchenfüßer ( Solenogastres ), about 250 species, Sea (Deep Sea )
• Chitons ( Polyplacophora ), about 600 species, sea (rocky coastal area)

The following five classes are summarized as shell molluscs ( Conchifera ) and probably form a monophyletic unit. They are distinguished by the characteristic mollusc shells.

• Contracts Manager ( Monoplacophora, rarely Tryblidia ), about 27 species, Sea (Deep Sea )
• Snails ( Gastropoda ), specified types of figures can vary significantly depending on the author ( 50,000-150,000 ), sea fresh Country
• Kahnfüßer ( Scaphopoda ), about 350 species, sea
• Mussels ( Bivalvia ), about 8000 species, sea fresh water
• Cephalopods ( Cephalopoda ), about 786 species, sea

Contracts Manager ( Monoplacophora ) seem to be the sister group of cephalopods ( Cephalopoda ), but not other shell molluscs ( Conchifera ).

For this purpose are some extinct classes, with their assignment to the molluscs is partly disputed; often mentioned in this context:

• † beak Schaler ( rostroconchs )
• † Helcionelliden ( Helcionellida )
• † Tentakuliten ( Tentaculitoidea or Cricoconarida )
• † Hyolithiden ( Hyolitha )

The phylogenetic relationships of the above groups are currently still in the river. Currently about half a dozen different pedigrees ( cladograms ) Total will be discussed only for the surviving classes, each supported by one or the other findings. Below is a cladogram is presented which is based on two in 2011, published in the journal Nature studies.

Chitons ( Polyplacophora )

Schildfüßer ( Chaetodermomorpha )

Furchenfüßer ( Neomeniomorpha )

Cephalopods ( Cephalopoda )

Contracts Manager ( Monoplacophora )

Kahnfüßer ( Scaphopoda )

Mussels ( Bivalvia )

Snails ( Gastropoda )