Monday, May 12, 2008

Gastropod shell, body and radula

Reactions:  
Today a brief overview of the most important Gastropod terms and vocabulary will be given together with short explainations. We will start with the shell of the Gastropod which is the most commonly preserved fossil remain of a Gastropod (Snails).

The shell is usually composed of the mineral Aragonite. The shell walls can be simplified into three layers called, from outside to inside: Periostracum, Ostracum and Hypostracum.
  • Periostracum: a thin organic layer which forms the outside of the Gastropod shell and protects it from dissolution
  • Ostracum: layer of calcium carbonate prisms orientated vertically to the surface held together by a proteine. It forms inbetween the fresh Periostracum and the mantle.
  • Hypostracum: thin, inner-most layer of horizontal calcium carbonate platelets covered by Nacre on the most inner layer.
Shell shapes can be generalised into three different types, excluding those Gastropods that have lost their shell. These are cup- or cone-shaped like in Patella, planispiral like in Planorbis or trochispiral like in Turbo. Due to their asymmetric nature these shells posess a chirality. Meaning they can be either left-handed (sinistral) or right-handed (dextral). Most species are dextral, few sinistral, some may have individuals of either.



The common features of a shell are:
  • Apex, the tip of the shell.
  • Columella, a spiralised cone in the centre.
  • Aperture, the only opening of the shell which may have different shapes.
  • Siphonal channel, an extension of the Aperture
  • Suture, the junction of whorls
Other important terms:
  • Protoconch: the larval shell
  • Teleconch: the adult shell
  • endogastritic: the spiral points away from the head
  • exogastritic: the spiral points towards the head
  • Torsion: rotation of the visceral mass, mantle and shell by 180° with respect to the head that brings the anus and mantle cavitiy above the head. Detorsion in some Gastropod species towards the side or complete detorsion.
  • Spirality: spiralling of the upper part of the visceral mass usually to the right
The Gastropod body is divided into four parts: head, foot, mantle (pallium) and visceral mass. With the exception of the Pulmonata (who have a pallial lung) all Gastropods have a pair of gills that can be reduced to one gill in more developed species.

An important feature in the classification of recent Gastropods is the Radula. There are several types of Radulas. It exists a trend in the development of the Radula to reduce the number of teeth. Few teeth can be an indication of a carnivor way of life. The most extreme case maybe being Conus (Danger: highly toxic!) with only a single tooth that is used to poison the prey. There are central teeth (Rhachis-tooth), teeth on the sides (Admedian-teeth) and marginal teeth.
  • rhipidogloss: most primitive type with most teeth
  • docogloss: fewer teeth, no marginal teeth
  • taeniogloss: few teeth per row
  • ptenogloss: numerous identical teeth
  • rhachigloss: mostly only central teeth and seldom marginal tooth
  • toxogloss: one tooth in the centre with hooks. Toxic!

2 comments :

cjvs97 said...

Can yuo tell me more about this shell I found nearly 30 yrs ago in the Philippines. Its as hard as a marble.

http://www.badongo.com/pic/3638941

Lost Geologist said...

To my own disgrace I do not know what kind of snail that is. It looks and sounds from your short description though that it may have been silified (meaning replaced by quartz or chalcedony). I recommend posting your image together with an extended description (where exactly in the Philippines? etc.) at this following great english language geology forum:

http://www.geologyrocks.co.uk/

I am active in it, too, and the people over there are all very helpful and from a wide variety of geological fields.