CEPHALOPODA, GASTROPODA

& other Molluscs

GASTROPODS


CLASSIFICATION & GEOLOGIC RANGES

 

Phylum Mollusca (Precambrian-Recent)

Class Gastropoda (Cambrian-Recent)

Subclass Prosobranchia (Cambrian-Recent)

Order Archaeogastropoda (Cambrian-Recent)

Order Mesogastropoda (Ordovician-Recent)

Order Neogastropoda (Cretaceous-Recent)

Subclass Opisthobranchia (Carboniferous-Recent)

Subclass Pulmonata (Carboniferous-Recent)

 


Class GASTROPODA

Gastropods, including such common forms such as snails, slugs, and whelks, occupy both marine and non-marine environments. Although many gastropods are herbivorous grazers, several groups are active carnivores able to drill through the skeleton of the luckless victim.

Most of the gastropods are classified on the characteristics the gill structures and other soft-bodied features. Few distinguishing characters of the univalved shell are used in classification as many are the result of convergent evolution. Although the differences in the shell form may be difficult to recognize, different morphologic groups can generally can be differentiated on characteristics of ornamentation, shell shape, and aperture. The shell of many gastropods can either be external or, less commonly, internal. The difference can often be deduced by the luster of shell material and the presence of other features such as deviations of a structural shell form.

Gastropods are radulate organisms with a torted body (e.g., the body is rotated 180 so that the anus is above the head. Gastropods typically have a helical coiled univalved shell whose opening (aperture) may be closed by an operculum. Another feature which is useful in discriminating among groups is the structure called the selenizone which can be expressed as either a series of holes as in Haliotis, or as a groove along the periphery which is often seen as a sharp bend in the growth lines of the shell. Other features such as ribs or the siphonal canal may also be important.

Although the shape of gastropods can be described in terms such as "high-spired", "low-spired", or "cap-shaped", many of the shape characteristics can easily be described mathematically by four parameters S, W, T, and D.


Figure 1 - Gastropod Coiling Parameters

Where:

S

shape of generating curve (loosely defined as the ratio between aperature height and width)

D

distance of generating curve from the axis of rotation

W

rate of expansion of generating curve

T

rate of translation along the coiling axis

Modified from McRoberts (1998)


Subclass Prosbranchia

The distinguishing shell feature among prosobranch gastropods is that they are all either cap-shaped or they are helically coiled.

Order Archaeogastropoda. Many archaeogastropods have an identifiable selenizone (except for some trochids) in addition to an operculum. The shell of archaeogastropods can be either internal or external. This group is exclusively marine. Most are turbinoform, but others may be high spired, cap-like as in recent limpets, or other shapes. Many of the cap-like archaeogastropods have a small hole in the apex of the cap which is a modified selenizone. The selenizone of other species of archaeogastropods may be a series of holes such as in Haliotis. As mentioned earlier, the group of involute univalves with a selenizone called bellerophons, (especially those with unpaired muscle scars), may be regarded as Archaeogastropods. Examples of bellerophons are provided.

Order Mesogastropoda Mesogastropods do not have a selenizone. Their shell can be either internal, which commonly have a slit like aperture, or external, in which round or ovid apertures are common, some of which have a lip. A very common group includes the turritellids, a high-spired group typical of post-Jurassic. Another common example of mesogastropods are the slipper shells, belonging to the genus Crepidula. Other forms, such as the filter feeder Vermicularia, often become uncoiled (or vary coiling parameters) during ontogeny. Of special note is the common low spired Polynices, who is an active carnivore who drilled holes in the shells of other molluscs.

Order Neogastropoda Neogastropods do not posses a selenizone, yet they typically have a siphonal notch or canal and elongated and non-circular, and commonly slit-like, apertures. Typical examples of this group occur in the common whelks and others with strong ornamentation. - Although other forms such as Conus are also quite common.


Subclass Pulmonata

Pulmonates can have an external or internal shell, or the shell may be absent (e.g. terrestrial slugs). Many pulmonates are terrestrial or live in lacustrine environments. The pulmonates can typically be recognized by their often thin shells and distinct shell morphology which is typically conispiral and rather bulbous. Furthermore, many pulmonates have a well defined aperture lip as in Helix. Here are several other examples.


Subclass Opisthobranchia

Most Opisthobranch gastropods are marine plankton and lack a mineralized skeleton. Members that do have a skeleton, including the pteropoda, are usually small and either cone-like or variable shaped because their shell is interior. There are no examples of pteropods and other opisthobranchs in the laboratory.


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