Class Gastropoda: Hard part morphology of the Gastropoda shell

Morphology of the Hard Parts of Gastropoda Class:
The gastropod shells are univalved as they consist of only one valve. Usually the shells consist of a long tube which is broad and open at one end and gradually tapers towards the other end. However, in the case of Patella, it occurs as a hallow cone. The average diameter of a gastropod shell is 2.5 cms and its length varies between 2 to 15 cms. However, microscopic gastropods (pteropods) have also been recorded. Compositionally, the shells are aragonitic calcareous.
The shell is coiled spirally into a screw like structure and each of the individual coils is described as a whorl (fig. 24.1).

characteristics of a typical Gastropod.
The shell is composed of many whorls. All the whorls are in contact with each other and the dividing line between two whorls is the Suture line. Siliquaria and Vermetus have whorls which are not in contact with each other. The whorls increase in diameter from the apex to the base and the broad end of the shell is called as the body whorl which has an opening called the aperture. The closed posterior end forms the protoconch. All the tapering whorls except the body whorl together form the spire of the shell.

FORMS OF GASTROPODS :
The forms of the gastropods are described taking into account the arrangement of the whorls, the spiral angle, the number, shape of the whorls, the size of the last whorl and its relation with the spire.
The different forms of yastropods are described below with illustrations Fig. 38.6.
(a)Advolute : The whorls are found to be in contact but not embracing, e.g., Planorbis.
(b)Evolute : These are loose-coiled shells where the whorls are not in contact, el., Ecculiomphalus.
(c)Involute : These shells have their outer whorls strongly embracing the inner whorls, e.g,, Owenella.
(d)Discoidal The shape of these shells is like a disc or wheel. The spire is extremely short and flat, e.g., Planorbis.
(e)Trochiform : The shells are conical in shape with a characteristic flat base. The sides of these shells are evenly confluent and are almost straight, e.g., Trochus.
(f)Turbinate : The shells are conical in shape with a characteristic rounded base, e.g., Turbo.
(g)Turreted : The shells have numerous whorls which increase in diameter evenly towards the base, e.g., Turritella.
(h)Fusiform : These shells are spindle shaped, thickest at the center and pointed at both the ends, e.g., Fusus.
(i)Cylindrical : The shell is roughly cyrindricai I'm shape. The whorls are nearify equal in diameter except the first few initial whorls, e.g., Pupilla. (hence, also called as Pupaeform).
(j)Globular : The spire of these shells is small and short while body whorl is large and' rounded, e.g., Natica.
(k)Biconical : The spire and the base of the body whorl together possess a conical shape, e.g., Conus.
(I) Platelliform : These are non-coiled shells which are low cap-shaped, e.g., PlateIla.

In some gastropods, the peristome is unbroken or entire and the shell is then described as holostomatous, e.g., Turbo (Fig. 38.3 a). However, in many gastropods the peristome forms a canal-like extension towards the anterior end and such shells are then described as siphonostomatous e.g., Fusus (Fi.g. 38.3 b). Some gastropods like Cyprea of the siphonostomatous type exhibit both anterior and posterior siphonal canals.

Based on the type of coiling, gastropods exhibit either dextral or sinistral type of coiling. To illustrate this, hold the shell ;in such a way. that the apex (posterior end) is pointed away from the observer. In this position, if the aperture is placed on the right hand side then the coiling is said to be dextral e.g., Turbo and Conus, where as the coiling is said to be sinistral, if the aperture is placed on the left hand side e.g., Physa (Fig. 38.4 a and b).
The inner parts of the whorls are often fused to form a solid axial pillar which extends from the apex to the base of the shell and is called the Columella (Fig. 38.5). In a cross section of the shell, the columella appears like the back bone strengthening the shell. The' shells are termed imperforate when the solid columella does not open at the base e.g.,
Turritella (Fig. 38.5 a). In some gastropods, however, the inner parts of the shell are not fused but have hollow tube which opens at the base of the shell. This opening is called the umbilicus and the shells are described as perforate, e.g., Natica (Fig. 38.5 b).

Drnamentation
The surface features of the gastropods commonly include varying types of colouration and sculptural markings which make these shells the most sought after objects on the beaches. The ornamentation consists of features such as tubercles, knobs, spines, ribs, nodes, granules or pits. These may be arranged symmetrically or separately.