What Kind of Shell is It? -- a Snail? --a Clam? --Something Else?

The Five Principal Classes of Shells

Class_Gastropod2.JPG (12636 bytes) Class _Bivalve.JPG (20875 bytes) Class_Chiton.JPG (16045 bytes) Class_Cephalopod.JPG (16358 bytes)
(Tusk Shells)

Click on the shell picture, above, for the appropriate shell directory and pictures.

Mollusc Overview

General. All of the shells above are inhabited by molluscs --invertebrate animals with an unsegmented body, that generally consists of a head, foot, visceral mass, and hanging flaps of the body wall that constitute the mantle. Zoologists group them in the phylum, Mollusca. Most live in the sea or fresh water and use gills for respiration, although terrestrial and some freshwater snails breathe by a rudimentary lung. Molluscs have no internal supporting skeleton. Instead, the shell usually serves to protect the body against the outside world. It is a product of secretion by the mantle, and it is deposited in several separate layers. Different types of molecule make up each layer, and depending on the type of secretion, the shell can be thickened, hardened, enlarged, or if necessary repaired by the overlapping mantle.

The molluscs might well be considered the ultimate examples of "morphing," both in regard to details of their body plan and details of the shell, itself.  Nevertheless, the basic body plan is one of front-to-back bilateral symmetry, with well defined nerve ganglia, gills, a blood circulatory system, as well as digestive, reproductive, and excretory organs (see, for example, Ruppert & Barnes).  Most of the eight classes of mollusc have existed since the Cambrian period, 500 million years ago.

Within the various classes, evolutionary modifications of  the internal organs and other features are quite extensive. Principal changes generally reflect the mode of feeding, predatory behavior, and habitat. For example, a head region with a rasping tongue (the radula) and sensory organs are usually well developed among snails, but not in bivalves. In most classes, the dorsal ("back") to ventral ("front") muscles are very strong, but adapted to how the shell is used by the animal. Reproductive features vary widely within as well as across the various classes, families, and genera. They range from molluscs that simply liberate egg & sperm to water, to those that have "penis" & "oviduct" counterparts for internal fertilization. 

Most  molluscs, and particularly the snails, have chemosensory organs for taste (the osphradium), organs for equilibrium orientation (the statocysts), and at least primitive eye spots. Some --extending across all classes-- also have well developed eyes (see, for example, Pecten, Chiton, or Strombus eyes).

Snails. Among most snails, torsion (and twisting) of the body mass is a hallmark. The effect of torsion is to twist the nerve cords and internal organs and to rotate the anus toward the head end. Torsion serves a protective purpose by allowing the animal to withdraw its vital mass into the shell, and it permits a more efficient use of the mantle cavity for separating waste exit and nutrient intake streams. Like snails, bivalves, tusk shells, and cephalopods all exhibit varying degrees of torsion, as well. Chitons do not.

The cap shell mollusc, Neopilina galatheae (Class: Tryblidia or "Monoplacophora") shows a complete absence of torsion and a straight-line rather than a rotated gut. It is an unusual living species, first discovered in 1957 and  thought to be representative of many now extinct snails. From the view point of evolution, its body plan is considered ancestral or likely to have been very common. 

All modes of feeding are seen among snails, but many are aggressive carnivores. Some of the carnivores can even be dangerous (see for example, cones, a lethal weapon).  Others, in contrast, may live simply,

by scraping algae off rock with their radula, or in a few species, by filter feeding on small detritus, as do clams. The higher taxonomic orders have been shown to exhibit some degree of intelligence (see nervous system).

Bivalves. The hallmark of most are modifications for a sedentary lifestyle that depends on filter feeding. The nervous system is correspondingly rather simple, in comparison to the gastropod nervous system. The gills, typically, have become elaborately modified for filtering out food particles and folds of the mantle (palps) assist in transferring mucous-trapped particles to the mouth. Bivalves are generally not carnivorous, and the internal organs associated with carnivorous predation are absent. Nevertheless, like most aspects of biology, some exception usually turns up; e.g., the tiny clam, Poromya granulata. It uses a hood-like extension of its inhalant siphon to capture small crustaceans alive. Thus, it lives as a carnivore.

Cephalopods. These molluscs show particularly advanced specializations for a fully carnivorous lifestyle, which include adaptations for both high speed swimming and eluding predators. The mantle is modified for powerful jet propulsion with rapid control over direction of movement. A shell is not needed for protection and is generally absent. However, a few species retain vestigial remnants of a shell completely enclosed within the mantle. Adapted, as they are, for rapid predation, cephalopods have excellent bilateral vision. Their cerebral ganglia are highly developed, and these molluscs have been shown to exhibit intelligent behavior. Nevertheless, small cephalopods comprise a major part of the food web of larger fish and whales.

Species & Species Differences: In the study of molluscs, species are generally considered to be evolutionarily stable, except to the extent that natural selection has caused a permanent change. Recent research studies are calling into question the idea that apparent species differences are always to be explained in terms of neo-Darwinian survival. Phenoplastic switch capability (see Species & Speciation) is known to occur across many phyla and has indeed been observed in the mollusca, as well; for example, the Donacidae, the Neritidae, and the marine and freshwater molluscs of the Physidae families.

Other Classes. In addition to the mollusc classes identifiable by the five shell types pictured above, several additional classes also exist (see below). Morphological developments in each of the eight classes, below, reflect unique evolutionary specializations. Arrangements of these classes and other taxa in the classification system (e.g., Gastropod Classification, Bivalve Classification, or Cephalopod Classification) attempt to parallel the progressive --or occasionally retrogressive-- nature of the specializations that have occurred. For more information, see the appropriate  family pages of this web site.

All Classes in the Phylum Mollusca:



Natural History Museum > Archerd Shell Collection > Shell Classes