Characteristics of Phylum Mollusca
1.- Dorsal body wall forms pair of folds called the mantle, which encloses the mantle cavity, is modified into gills or lungs, and secretes the shell (shell absent in some); ventral body wall specialized as a muscular foot, variously modified but used chiefl y for locomotion; radula in mouth.
2.- Live in marine, freshwater, and terrestrial habitats.
3.- Free-living or occasionally parasitic.
4.- Body bilaterally symmetrical (bilateral asymmetry in some); unsegmented; often with definite head.
5.- Triploblastic body.
6.- Coelom limited mainly to area around heart, and perhaps lumen of gonads, part of kidneys, and occasionally part of the intestine.
7.- Surface epithelium usually ciliated and bearing mucous glands and sensory nerve endings.
8.- Complex digestive system; rasping organ (radula) usually present; anus usually emptying into mantle cavity; internal and external ciliary tracts often of great functional importance.
9.- Circular, diagonal, and longitudinal muscles in the body wall; mantle and foot highly muscular in some classes (for example cephalopods and gastropods).
10.- Nervous system of paired cerebral, pleural, pedal, and visceral ganglia, with nerve cords and subepidermal plexus; ganglia centralized in nerve ring in gastropods and cephalopods.
11.- Sensory organs of touch, smell, taste, equilibrium, and vision (in some); the highly developed direct eye (photosensitive cells in retina face light source) of cephalopods is similar to the indirect eye (photosensitive cells face away from light source) of vertebrates but arises as a skin derivative in contrast to the brain eye of vertebrates.
12.- No asexual reproduction.
13.- Both monoecious and dioecious forms; spiral cleavage; ancestral larva a trochophore, many with a veliger larva, some with direct development.
14.- One or two kidneys (metanephridia) opening into the pericardial cavity and usually emptying into the mantle cavity.
15.- Gaseous exchange by gills, lungs, mantle, or body surface.
16.- Open circulatory system (secondarily closed in cephalopods) of heart (usually three chambered), blood vessels, and sinuses; respiratory pigments in blood.
Integrated principles of zoology / Cleveland P. Hickman, Jr. ... [et al.]. – 14th ed. p. cm.