How would you recognise a gastropod?
This is easy as the animal is still in existence and its recognised by its soft parts. This gives us a problem when considering fossil examples, as the soft tissue is usually not preserved.
How do modern gastropods live?
The gastropod has a recognisable head with tentacles for sensing its surroundings. This marks the anterior of the animal. It crawls along on a muscular foot with the rest of its soft parts stuck in the shell. To manage this the guts twist 180*, which is a biological distinguishing characteristic of a gastropod . When threatened, the whole body can be withdrawn into its shell.
As the gastropod grows it does so by laying down calcium carbonate on the lips of the aperture. The outer lip grows faster than the inner lip to achieve coiling as well as increase in size. Each complete coil is called a whorl. The soft body occupies the last whorl , which can therefore termed the body chamber. The rest coils up to the apex to make up the spire.
Types of coiling
Most of the coiling is right-handed or dextral, which ends up with a view of the open end, or aperture on the right. coiling to the left is known as sinistral. A good way to distinguish a gastropod from a cephalopod, is that there are no chambers within the shell. Not all gastropods show helical coiling, as some, notably freshwater species, are coiled in a plane (planispiral) and some strange cephalopod's show helical coiling.
Mode of life
The shape and construction of the shell is a good indication of the mode of life. Forms with a large siphonal canal have this to separate inhalent and echalent currents. Thin delicate shells, as seen in freshwater species, are an indication of low energy, conversely a thick, ribbed or ornamented shell indicates high energy.