Brief notes on the Morphology Gastropoda

Gastropoda constitute an important group of invertebrates among the mollusca. The term gastropoda has been coined from two Greek words, gastros (stomach) + podos (foot), which refer to the presence of foot, on the ventral part of the animal.

It is remarkable to observe the gastropods as group has been constantly growing from Cambrian to present day with negligible history of extinction. These are mostly marine animals living in shallow seas, though some are terrestrial and a few others live in fresh water environment.

Morphology:

The animal with its soft parts lives inside a cone shaped shell which in a majority of cases is spirally coiled. The soft parts include the broad flattened muscular foot, a distinct head with one or two pairs of tentacles, a pair of eyes, and mouth along with teeth like structures and radula (file-like tongue), digestive, circulatory, respiratory, nervous, urinary and genital systems. The soft parts are enclosed in a bag formed by the mantle which in turn secretes the outermost univalve shell.

The Gastropod shell is a long tubular cone like structure, which is generally coiled. The coiling may be helicoidal like a screw or may be planispiral. Each coil of 360 degrees is known as a whorl. The number of whorls varies widely in different forms. In a majority of cases the whorls are in contact with each other while in a few cases they are separated from each other.

The line of contact of the whorls is termed as suture line. In the embryonic stage the shell of the animal starts as a small pointed structure known as protoconch. As the animal grows older, the whorls increase in size as well as in number by addition of calcareous substances secreted by the mantle. All the whorls together from below the protoconch, except the last whorl, constitute the spire of the shell.

The last whorl in which the animal lives is generally larger than rest of the whorls and is known as the body whorl.

The pointed part of the shell is known as apex and the part of the shell opposite and farthest to that of the apex is termed as base. Basing on the number and size of the whorls, the nature of the spire in different forms varies widely. It may be long, short, horizontal (flat) or depressed.

The angle produced by drawing two tangents on either side of the shell from the apex to the base is known as spiral angle. The spiral angle in different forms varies depending upon the shape of the shell. The opening of the shell is known as aperture. In clockwise or dextral coiling, the aperture comes to the right hand side of the observer if the shell is held upright position while in anticlockwise or sinistral coiling the aperture comes to the left hand side.

The inner part of the whorls fuse together to form a pillar like structure extending from the apex to the base is called the columella and the shell with a columella is known as imperforate. In other forms, the inner part of the whorls instead of fused together are left as a tube-like hollow space, which extends from the apex and opens at the base of the shell is called as umbilicus. A shell with an umbilicus is known as perforate.

The gastropod shell is a single chambered conical tube-like structure closed near the apex and opens near its base. In exceptional case it may be multichambered. The shape and size of the aperture vary in different forms and the operculum, a horny plate which closes the aperture varies accordingly.

The operculum is attached to the dorsal part of the foot and closes the aperture when the animal withdraws into its shell. It is rarely preserved as fossil. The margin of the aperture is named as peristome. The inner side of the peristomal margin is known as inner lip while the outer side is called as outer lip.

The outer lip may be rolled outwardly or inwardly, when it is termed either as reflected or inflected. The peristomal margin may be provided with two notches, one at the anterior end and the other at the posterior end. These are known as anterior or siphonal canal and posterior or anal canal respectively. Such forms are known as siphonostomatous. Shells without any break in the peristomal margin are known as holostomatous.

The shells of the gastropod are ornamented with different ornamental elements like axial and spiral coastae, smooth, pitted and reticulate sculptures, needles and spine. These ornamental features are arranged either parallel or transverse to the suture line. Many gastropod shells are decorated with shades of different colors, which are seldom preserved in fossils.

Depending upon raised or depressed spire, number and shape of the whorls, nature of the spiral angle and size of the last whorl, a variety of forms of gastropod shells have been observed. Some of them are given below.

(i) Discoidal – Planispirally coiled. All the whorls are in one plane.

(ii) Turbinate – Spire angle acute, spire sharp, base flat.

(iii) Trochiform – Spire acute, body whorl is globular with convex base.

(iv) Turreted – Spire long with numerous whorls, sharp and acute apex, body whorl is small compared to the long spire.

Body whorl large with the aperture as long as the body whorl, spire is short.

Shell spindle-shaped, body whorl widest at the middle of the shell and tapers interiorly with long siphoned canal. Convex apexes. After the first few whorls, the diamel of rest of the whorls remain same. Spire sharp and short. Body whorl is globular and large. Spire concealed by the last whorl. Aperture is as long as the shell and with crenulated lips. Shell cap-like with sharp and acute apex.

The Gastropods are a successful group of invertebrates with wide geographical distribution and long geological range. Hence, their fossils are found preserved in most of the rocks deposited beginning from Cambrian to the recent period.