Phylum Mollusca
Protostomes. Schizocoelous body cavity restricted to the blood sinuses. Body monomeric, highly variable in form. Head with eyes and sensory tentacles, radula in mouth for scraping. Large, muscular foot. Posterior mantle enclosing mantle cavity and secreting protective external shell of protein (concholin) reinforced with calcareous spicules or 1 to 8 calcareous plates. Gas exchange by one or more pairs of ctenidial gills. Open circulatory system. Paired kidneys connected to pericardium. Nervous system a circumesophagael ring. Typically 1 pair of gonads discharging into mantle cavity. Spiral cleavage. Indirect development through trochophore and veliger larva.

Class Monoplacophora
Thought extinct until 1972. Single, conical shell. weakly muscled, circular foot surrounded by extensive mantle cavity with 5-6 pairs of monopectinate gills. No sensory tentacles (except around mouth) or eyes. Well developed radula.

Class Polyplacophora
The chitons. Oval to elongate. Dorsoventrally flattened. Eight overlapping shell plates. Mantle developed into thickened girdle that can bear spines and/or bristles, and can overgrow the shell plates. Head poorly developed, covered by anterior girdle. Eyes and sensory tentacles lacking. Radula large. Large, muscular foot makes up most of ventral surface. Six to 88 pairs of bipectinate ctenidia.

Class Gastropoda
Visceral hump rotated 180 degrees during development (torsion), resulting in anterior opening of mantle cavity and anterior discharge of anus and kidneys. Well developed foot. Well defined head with eyes, sensory tentacles and a radula. Head and foot retractable into a single, coiled shell. Plesiomorphically 1 pair of bipectinate ctenidia. Gonochoristic.
Cypraea sp. - cowrie (subclass Prosobranchia, order Mesogastropoda, superfamily Cypraeacea)
Aeolidia sp. - sea slug (subclass Opisthobranchia, order Nudibranchia)
Lymnaea spp. - freshwater pulmonate snail (subclass Pulmonata, order Basommatophora)
Helisoma spp. - freshwater pulmonate snail [1] [2] (subclass Pulmonata, order Basommatophora)
Limax sp. - land slug (Subclass Pulmonata, Order Stylommatophora)
Thais sp. - dog whelk eggs (subclass Prosobranchia, order Neogastropoda, superfamily Buccinacea)
Crepidula sp. - slipper snail [cleavage] [veliger] (subclass Prosobranchia, order Mesogastropoda, superfamily Calyptraeacea)

Class Bivalvia
Laterally compressed. Entire body enclosed within 2 lateral valves. Foot reduced in attached forms. No cephalization. Radula absent.
clam gill [wm] [cs]
'mussels' - marine mussel (left)(subclass Pteriomorphia, order Mytiloidea, family Mytilidae)
- Dreissena polymorpha - zebra mussel (middle)(subclass Heterodonta, order Veneroidea, family Dreissenidae)
- freshwater mussel (right)(subclass Pteriomorphia, order Unionoida, family Unionidae)
Unionidae - freshwater mussels ((subclass Pteriomorphia, order Unionoida)
unionid glochidia
Ensis directus - Atlantic jacknife clam (subclass Heterodonta, order Veneroidea, family Solecurtidae)
giant clam (subclass Heterodonta, order Veneroidea, family Tridacnidae)

Class Scaphopoda
Tusk shells. Tubular shell open at both ends. Cone-shaped foot. Captacula. Gills absent.

Class Cephalopoda
Raptorial pelagic molluscs. Foot divided into muscular arms. Propulsion for swimming generated by pumping action of mantle cavity. Shell absent or reduced. Where present, shell partitioned into gas-filled chambers producing buoyancy.
Loligo sp. - squid [juvenile] [adult]
Octopus sp.
Nautiloidea


Phylum Annelida
Segmented worms. Hydrostatic, metameric, schizocoelous coelom. Presegmental prostomium with ganglion and post-segmental pygidium. Outer epitheliem covered by cuticle and containing setae. Closed circulatory system. Spiral cleavage. Trochophore larva.

Class Polychaeta
Predominantly marine. Setae inserted in 2 discrete bundles in biramous, segmental parapodia.
Nereis sp. - errant polychaete [larva] [adult] [cs]
Aphrodita sp. - sea mouse
Chaopterus sp. - sedentary polychaete [1] [2]
Amphitrite sp. - sedentary polychaete
Serpula vermicularis - sedentary polychaete
Hydroides hexagonus - sedentary polychaete

Class Oligochaeta
Few, small setae. Predominantly terrestrial or freshwater (marine forms a secondary invasion). Simultaneous hermaphrodites. Locomotion dependent on completely septate segments.
Lumbricus terrestris - earthworm [cs1] [cs2]

Class Hirudinoidea
Oral and posterior suckers for attachment. Internal septa reduced or absent, resulting in development of coelomic sinuses. 33 segmental ganglia. External annuli not corresponding to metameric segments. Coelom filled with 'parenchyma-like' tissue. Hermaphroditic. Predatory.
leech


Phylum Echiura
Large extensile and contractile proboscis with lateral folds. Single, schizocoelomic cavity. Single pair of setae in anterior ventral region, and often seate surrounding anus. Closed circulatory system. Circumesophageal ring and ventral nerve chord without ganglia. Excretory system with up to 400 metanephridia. Gonochoristic. Spiral cleavage. 'Trochophore-like' larva.
[1] [2]


Phylum Sipuncula
Anterior introvert with mouth and circumoral tentacles. Muscular, complete, U-shaped gut (anus situated anterio-dorsally). No metamerism. Schizocoelous. Outer epithelium with cuticle, but no setae. No circulatory or respiratory system. Anterior 'brain', circumesophageal nerve ring, and ventral nerve chord without ganglia. Single or paired metanephridium. Spiral cleavage. Trochophore larva.
[1] [2] [3]
Phascolosoma gouldi