Gastropods

GASTROPODS

Term gatropoda means stomach (gastro) and foot (poda), so gastropods are characteristically consisted of stomach and a foot.

Morphology

Shells of gastropods are mostly dextral coiled with few exceptions like Physa and Planorbis in which shells are sinistrally coiled. See the image below.

Left side gastropod has sinistral coiling (aperture is on left side) while right side one has dextral coiling (aperture is on right).  [Source : Wikipedia]

Two sets of tubercles present called coastea.

Operculum closes the shell. It was lost with evolution.

Peristomal slit separates inhalant and exhalant siphons. Selenizone separates siphon from mouth.

Central axis inside the shell is called columella if solid, or Umblicus (perforate) if hollow.

Siphonal canal is present on anterier side while anal canal is present on posterior side.

Gastropods can have outlet for exhalent called Siphonstomatus. Movement of water in these forms are continuous as siphon is present. Examples are Murex, Voluta, Cerithium, Fusus, Patella, Bellerophone, Conus, Cyprea and Terebra. 

Holostomatus forms are unmarked by any feature. There is only single tube present. Ex. Turritela, Natica.

Torsion is characteristic feature in some gastropods. In this, mantle cavity lies above the head, i.e. change in original positions of organs above and below the neck.

_______

Classification of Gastropods

Prosobranch 

Torsion complete. Divided into 3 sub types

Archa-proso : Few primitive forms of cambrian.

Meso-proso (Ordo - recent) : Cyprea, Natica, Cerithium etc.

Neo-proso (Cretaceous - recent) : Murex, Voluta and Conus etc.

Opisthobranchia

Carboniferous to recent

Marine and detorsion.

Example is Pteropods.

Pulmonate 

Gills are absent. They use lungs for respiratory system.

These are freshwater or terrestrial.

Ex. Helix, Planorbis, Pupella.

____

Some Important Gastropods

Dextral forms

Patella : Unilateral symmetry absent, Conical or patelliform coiling, Jurassic to recent.

Bellerophon : It is prosobranchia but without torsion, Isostropic shell, Silurian to permian.

Natica : Globular shell, Triassic to present.

Sinistral forms

Planorbis : Discoidal shell, wide umbilicus, terrestrial, Jurassic to present.

Physa : Jurassic to present.

Naptunea : Large body whorl and short siphonal canal. 

_________________

Coiling in gastropods

Discoidal : Planorbis and Malurites.

Turreted : High spired. Turritela and Cerithium.

Cylindrical : Pupilla and Voluta.

Globular : Natica

Convolute : Earlier whorls are enclosed by final whorl. Crenulated lips present. Ex. Cypraea.

Auriform : Haliotis.

Conical : Parallel lips present. Patella, Conus.

Digitate : Aporrhais

___________

Evolution trends in gastropods

• Increase in tightness of coiling.

• Evolution of apex w.r.t to last whorl.

• Size of whorl increased.

• Loss of operculum.

• Development of new shell forms like torchiform, fusiform etc.

_____________