CEPHALOPODA, GASTROPODA

& other Molluscs

 

From Eldredge (1991)

 


INTRODUCTION

Molluscs are greatly varied in both morphology and life habits. They include familiar living forms such as clams, snails, the octopus, and squids. Several groups have great economic importance such as oysters and the blue mussels, and some groups such as garden slugs and the zebra mussel can be bothersome pests. In this laboratory you will become familiar with several of the molluscan groups, of which the cephalopods and gastropods are geologically the most important.

Most molluscs are mobile marine creatures, although some have invaded terrestrial environments. Molluscs have an external shell (single or bivalved) enclosing the mantle and visceral mass, a muscular foot which aids in attachment and/or locomotion, and a radula. The radula is not known from the bivalves (a group covered in the next lab), but is presumed to have been primitive in the bivalve ancestor.


CLASSIFICATION & GEOLOGIC RANGES

Phylum Mollusca (Precambrian-Recent)

Class Monoplacophora (Cambrian-Recent)

Class Amphineura (U. Cambrian-Recent)

Class Schaphopoda (Ordovician-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 Cephalopoda (Cambrian-Recent)

Subclass Coleoidea (Devonian-Recent)

Subclass Nautiloidea (Cambrian-Recent)

Subclass Ammonoidea (Devonian-Cretaceous)

 


Class MONOPLACOPHORA

The organ system of monoplacophorans are arranged in a series and are segmented (or pseudosegmented) leaving paired muscle scars on the shell interior. Some investigators suggest that a Paleozoic and early Mesozoic group called the Some fossils typically classed as bellerophons (here treated as gastropods), may have an untorted body plan. This has been deduced from the arrangement of muscle scars on the interior of the univalve bellerophons. Most other bellerophons, however, do exhibit torsion and should rightly be classified as true gastropods. Apart from the bellerophons (no muscle scars can be observed in our specimens) we have no monoplacophoran specimens.


Class AMPHINEURA

Amphineura (Polyplacophora of some texts and commonly referred to as the Chitons) are characterized by an elongated body with a head and bilateral symmetry. Polyplacophorans possess a radula. The soft bodies of polyplacophorans are surrounded by a muscular mantle girdle (cuticle) which has aragonitic spicules. Above the mantle cuticle, polyplacophorans have a dorsal shell made up of eight articulated plates or valves. Some recent examples of polyplacophorans are provided here.


Class SCHAPHOPODA

Scaphopods (commonly called tusk shells for obvious reasons) are a relatively minor class of marine molluscs. They can actively trap food particles within the sediment by use of their specialized tentacles. Scaphopods are characterized by a small univalved shell that is open at both ends. The larger anterior end is permanently embedded in the sediment. The smaller posterior end is opens near the sediment-water interface. See the examples of Recent and fossil Dentalium.


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GASTROPODA

CEPHALOPODA


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