Gastropod shell

As a snail shell or housing the spirally wound calcareous shell of the snail is called. It is connected to two muscles with the body of the worm and serves to protect the mollusk body from predators and injury.

Evolution of the auger housing

The auger housing was built in the course of a long, onset already in the Cambrian evolutionary process from calcareous shell plates and earlier molluscs. The shell is formed from it is all members of the Conchifera own. In contrast to the shells of the mussels ( Bivalvia ), Kahnfüßer ( Scaphopoda ) and cephalopods ( Cephalopoda ) is the shell of the snail, but always wound spirally with all the variety in shape and various external similarities to bowls this mollusc groups.

Construction and shape of the shell

The term " thread " includes the protoconch and the Teleoconch up to and including the penultimate intercourse. The last use of the mouth is called the base.

Protoconch

The shell is formed at the embryonic stage of the worm. The scale in this stage of development portion of the shell forming the tip (apex ) of the housing, from its further course it can differ significantly. The shape of the Embryonalgewindes or Protoconchs is a feature that plays a significant role in the identification of species (especially in paleontology, as are almost never get soft tissue). This part of the case is usually smooth, but can also be sculpted.

The protoconch can be:

• Orthostroph (also homöostroph ) = the embryonic whorls has the same winding direction as the Teleoconch ( the most common case )
• Heterostroph = with opposite winding sense ( much more rarely )
• Alliostroph = The embryonic whorls runs in relation to Teleoconch offset at an angle of 90 ° ( in this case " is " it virtually on the other thread)
• Paucispiral = with 1-2 whorls
• Multispiral = with three or more whorls.

Teleoconch

The following on the protoconch section, which occupies the greater part of the case is referred to as Teleoconch. The growth lines and other defining characteristics of the sculpture of the Teleoconchs usually occur on species-specific in very richly varied forms. See the explanations in the relevant sections below.

Internal structure of the shell

The snail's shell is constructed of several layers. From the inside out the layers are labeled: hypostracum, Ostracum and periostracum. The first two layers are made of aragonite, a calcareous (CaCO3). They form the stable " backbone " of the shell. Periostracum the other hand, which may also be referred to as the shell skin, consists of a complex protein known as the conchiolin (also called Conchin ). This organic protective outer layer of the shell can not be renewed in adult worm, as the seated at the mouth glands in which the Conchiolin is produced only work during the growth of the snail shell. However, snails violations of their shell from the inside out with the help of lime can repair Depositing gland cells, the " repair " but it has a rougher surface texture clearly visible mostly due to the lack of periostracum on.

When thin aragonite platelets are deposited on the inside of the shell, this inner shell layer ( the hypostracum ) cause dazzling effects of light refraction. This is known as nacre of the shell occurs with some salt and fresh water snails, especially in the abalone ( Haliotis ).

The spiral shape of the shell

The snail shell extends from the apex to mouth on its own axis, which is referred to as a spindle ( columella ) when the whorls touch. Otherwise, create a funnel- shaped cavity, the navel (umbilicus ). The reason for the emergence of this spiral shell shape is to be sought in the ontogeny of the worm: One side of the visceral sac is growing faster than the other, whereby a rotational movement occurs, which in turn determines the spiral form and the winding of the shell. This process is referred to as twist.

The shell of the snail can - usually but not always species specific - rechtsgewunden ( dextral ), as in the great majority of snail species, or linksgewunden ( sinistral ) be. If the upright ( up to the apex) of the housing is facing the viewer mouth left, it says linksgewunden, rechtsgewunden in the reverse case. Left-handed snails are popularly referred to as a worm kings.

More shell features

Size, shape and pattern of the shell are often species specific and thus in the determination of significance.

Size

Aquatic living augers due to buoyancy in the water tend to be larger forms emerged as rural residents. In particular, under the sea snails very large species with some very serious cases before, such as occurring in the maritime area between Indonesia and Australia to 1 m case height attained Large proboscis worm ( Syrinx aruanus ). On the other hand, are among the recent worm encountered forms whose body size significantly below one millimeter is located (for example, the members of the family Omalogyridae ).

Sculpting

The surface of the shell has a more or less distinct sculpture whose characteristics can vary greatly pronounced.

It consists in the simplest case, growth lines, resulting in the growth phase. The course of the growth lines on the body is described as follows ( view each case on the upright, ie with the apex facing up housing overlooking the estuary - as in the semi Cassis shown opposite granulata ):

• Orthoclin = straight line from top to bottom,
• Prosoklin = slightly curved from top left to bottom right,
• Opistoklin = slightly curved from top right to bottom left,
• Prosocyrt = boomerang -shaped with the convexity to the left,
• Opistocyrt = boomerang -shaped with the convexity to the right.

An anatomical feature which is closely related to the growth lines, the so-called varices (sing.: varix ). It is thickened axial ribs, which usually occur at regular intervals on the whorls and have been designed as a thickening of the outer lip during a quiescent state in the growth of the worm. It can be seen that the growth of the snail has taken place in spurts ( see illustration ).

In addition to growth lines and varices cracks, gouges, streaks, keels, ribs, knots, stings and swellings may occur on the shell surface. Run such features parallel to the suture, they are called spiral ornament. Very fine lines or furrows are lirae (sing.: Lira ) called. If the shell ornaments are (ie perpendicular to the suture ) arranged obliquely or vertically, it is called cross ornament.

The mouth

The mouth forms the end of last intercourse (body handling, basis). For simplicity it is structured in the following the opening edge in three areas:

• The parietal ( wall outlet ): This area lies immediately below the penultimate round of the shell ( ie the top of the mouth edge )
• The Palatalis ( palate wall ): The outer facing portion of the mouth edge.

Spindle wall and mouth wall running down one another and form a channel which is quite far pulled in some species. This channel is referred to as Siphonalkanal or spout. The rear (outer) part of the spout, the handle (also called a plate).

On the spindle edge, small beads can be located, which are called spindle or spindle wrinkles teeth. Spindle wrinkles may also be located inside the housing, then they are not visible from the outside with an intact shell.

Dish lid

Using a shell lid screws can close the mouth of its shell. The prosobranch is the so-called operculum, which has grown at the foot of the screw. Other cover types are the Epiphragma (mainly known from the Roman snail ), which is temporarily formed for the purpose of protection against freezing and dehydration of a body's secretions, as well as the clausilium the door snails ( Clausiliidae ), which is firmly attached to the shell, but not with the soft body. Only this latter form of the lid shell can be regarded as part of the screw shell.