Phylum Mollusca
Defining characteristics (from Pechenik, 2000)
“Although the mantle is a major molluscan characteristic, its role varies substantially in different molluscan groups.”
Mollusca constitute the 2nd largest animal phylum
Mollusc shell schematic
Vertical & tangential sections of Nautilus nacreous layer
Mytilus prismatic layer
Molluscs use the radula to scrape food off of solid surfaces
Although a countercurrent exchange system is not unique to molluscs, this is the first phylum to have this type of circulatory system.
Green arrows indicate flow of blood, yellow arrow indicates water flow.
Classes in the Phylum Mollusca
Class Polyplacophora: Shell forms as a series of 7 to 8 separate plates.
Internal anatomy of a chiton
Class Aplacophora: Cylindrical, vermiform body with the foot forming a narrow keel.
Class Monoplacophora: 1) 3 - 6 pairs of ctenidia; 2) 6 - 7 pairs of nephhridia; 3) multiple pairs of foot (pedal retractor muscles)
Class Gastropoda defining characteristics
Torsion in gastropods may take a short period of time to occur.
Operculum provides protection from predators and from desiccation. The entire animal hides within its shell. (Compare this figure to #12.9a on page 214)
Subclass Prosobranchia: Mantle cavity generally anterior, due to torsion.
Subclass Opisthobranchia: Mantle cavity lateral or posterior due to detorsion, or lost, as in the Abalone.
The subclass Opisthobranchia also includes colorful marine nudibranchs, which advertise their poison or poor taste to predators.
Subclass Pulmonata: Mantle cavity highly vascularized and otherwise modified to form a lung (includes snails & slugs).
Class Bivalvia: Defining Characteristics
PPT Slide
Some bivalves attach to substrate by byssal threads
Class Scaphopoda defining characteristics
Class Cephalopoda defining characteristics
Orientation of cephalopod body is turned, relative to other groups.
Shell divided by septa, with chambers connected by the siphuncle
Foot modified to form flexible arms and siphon
Cephalopods possess eyes and visual systems which, in some ways, resemble those of vertebrates.
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Other information: Review of Mollusca