The Five Principal Classes of Shells

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 certain terrestrial 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.

Snails. Among most snails, torsion (and twisting) of the body mass is the 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 circulation of waste and nutrient 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 ancestral and likely was once 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 by scraping algae off rock with their radula, or in

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 five principal classes of mollusc, pictured above, several additional classes exist (see below). The morphological developments in each of these lineages reflect unique evolutionary specializations in response to ecological habitat and mode of living. The arrangements of taxa, i.e., categories in the classification system (e.g., Gastropod Classification, Bivalve Classification, or Cephalopod Classification) attempts to parallel the progressive --or occasionally retrogressive-- nature of these specializations, which have occurred across all molluscan families and genera. More information will be found on the appropriate  family pages of this web site.

All Classes in the Phylum Mollusca:

• Class: Caudofoveata (Aplacophora)
• Class: Solenogastres (Aplacophora)
• Class: Polyplacophora
• Class: Tryblidia (Monoplacophora)
• Class: Bivalvia (Pelecypoda)
• Class: Scaphopoda
• Class: Gastropoda
• Class: Cephalopoda