It is useful to know a few simple terms that describe the different parts of seashells. This special terminology defines the particular features of a shell that distinguish one species from another. It also helps to know these words in order to understand accounts of the development of the creatures, their biology and life history.
I am starting with a few names of basic parts in a gastropod mollusc shell. The example illustrated is the Common Whelk – Buccinum undatum Linnaeus. A gastropod shell is like a long, tapering tube or cone that has been coiled up. The top of the coiled shell, where the diameter of the tube is at its smallest, is called the apex of the shell.
Each complete turn of the shell tube, each coil, is known as a whorl. The areas where the whorls touch and join each other are called sutures. As the living animal grows, the diameter of its shell increases. Most of the body of the animal is housed in the last large whorl, the body whorl, leading to the mouth or aperture of the shell. Typically the snail can protrude from the aperture or withdraw into it. The whorls above the body whorl are termed the spire. The spire can be short and squat (like in a Necklace Shell) or drawn out to form a point (as in the Common Whelk).
The flesh of the animal is covered by a layer of tissue called the mantle. The mantle makes the shell. The shell is made up of three layers. The inner and middle layers of shell are composed of calcium carbonate crystals arranged on a framework of an organic, protein-like material called conchyolin.
The smooth inner or nacreous layer of the shell is continually secreted by the overall surface of the mantle with which it is in continuous contact. In some species of mollusc this is a layer of iridescent ‘mother of pearl’. The thin sheets of crystalline calcite that make up this inner layer lie more or less parallel to the surface. The inner layer grows in thickness throughout the life of the animal.
Around the aperture, the edges of the mantle produce the middle layer of the shell which comprises crystals of aragonite lying at right angles to the surface. This layer stops getting bigger when the mollusc becomes adult.
The outermost layer of the shell is the periostracum. This is also formed by the edge of the mantle and stops growing on sexual maturity. It has no calcium carbonate crystals and is entirely made of conchyolin. It is thin and easily worn away and rarely seen in mature gastropods.
The outer calcareous layer of the shell has patterns and textures that are typical of the species. Spiralling around the shell of the whelk, along the length of the ‘tube’ as it coils, is a series of narrow ridges and grooves called striations. Running diagonally across the striations, and curving right over each whorl, are thick ribs. On a much finer level, very fine growth lines loop the circumferance of the ‘ tube’ of the shell, and intersect the striations.
The next posting on Jessica’s Nature Blog will show photographs and detailed close-ups of the patterns and textures made by growth lines and sculpturings on the outer surfaces of Common Whelk shells. It will also further illustrate some of the features mentioned here.
Revision of a post first published 6 April 2010
COPYRIGHT JESSICA WINDER 2011
All Rights Reserved