Mollusca: Study Notes

Diversity: 90,000 species (Hickman et al. 2017)
Classes: 8 total:
- Caudofoveata
- Solenogastres
- Polyplacophora
- Monoplacophora
- Gastropoda
- Bivalvia
- Scaphopoda
- Cephalopoda

Versatility: Most versatile body plan among animals.
Body Size Range: From microscopic to 20 meters; most are <10 cm.
Habitat: Marine, freshwater, and terrestrial environments.
- Only Gastropoda and Bivalvia are found in freshwater.
- Only Gastropoda are found in terrestrial areas; must be moist and soil must contain calcium.

Body consists of two principal parts:
- Head-Foot: Active region containing sensory, feeding, and locomotor organs.
- Visceral Mass: Contains most internal organs embedded in tissue.

Mantle: Secretes shell or serves as body covering.
Shell: Can be external or internal (made primarily of calcium carbonate).
Radula: Specialized feeding organ (not present in bivalves and solenogastres) resembling a tongue with rasping teeth.
Foot: Used for locomotion; can be modified for various functions.

Structure: Tongue-like organ used for feeding.
- Present in all molluscs except bivalves and solenogastres.
- Contains many teeth (up to 250,000); new teeth are produced continuously.
- Supported by a structure known as odontophore (carries radula).
- Functions like a conveyor belt, transporting food to the digestive tract.

Foot Functionality:
- Present in all molluscs, used for various locomotion methods:
- Creeping movements (e.g., snails).
- Slime trail for movement.
- Digging into substrates.
- Attachment via byssal threads in some species.
- Some molluscs have modified feet as proboscis.

Contains digestive, respiratory, and reproductive organs.
- Mainly relies on ciliary tracts for functioning.

Mantle:
- Composed of epidermis, may have sensory organs and glands (secrete mucus, shells).
Mantle Cavity:
- Houses respiratory organs; offers protection in some species.

Composition: Mostly calcium carbonate; can be quite variable in form/structure.
Composed of three layers:
- Periostracum: Outer layer, thick in freshwater; thin or absent in marine.
- Prismatic Layer: Dense middle layer.
- Nacreous Layer: Inner glossy layer, secreted throughout life.

Majority have folded, ciliated gills (ctenidia); also aid in feeding for bivalves.
Some molluscs can respire through skin.
Terrestrial snails (pulmonates) often lack gills; modified mantle serves as lung (contains pneumostome for breathing air).

Most have an open circulatory system: blood not entirely contained within vessels.
- Adequate for slow-moving species.
Cephalopods possess a closed circulatory system:
- Features a simple heart with few vessels (two auricles, one ventricle).
- Coelom is reduced and becomes hemocoel around the heart.
- Blood contains both oxygen-carrying cells and white blood cells.
- No thrombocytes present.
- Haemocyanin (copper-based, blue color) and Hemoglobin (iron-based, red color) vary in presence.

Modes of feeding include detritus feeders, burrowers, grazers, and filter feeders.
Digestive organs embedded in visceral mass; complete digestive system present:
- Foregut: Receives/prepares food; includes mouth, radula, salivary glands, esophagus.
- Midgut: Storing and crushing region (stomach and digestive glands).
- Hindgut: Absorbs nutrients/formation of feces (intestine).

Typically involve one pair of metanephridia; the term 'kidney' is applied, but they differ from vertebrate kidneys.
Some excretion occurs through the body wall and gills.

Central nervous system consists of a ring of ganglia in the head area, with paired ganglia extending throughout the body.
Overall simple compared to vertebrates; however, cephalopods exhibit advanced reasoning and learning capabilities.

Majority are dioecious; however, many gastropods are monoecious.
Marine forms produce larval stage called trochophore, with some developing a secondary larval form called veliger.
Cephalopods undergo direct development, bypassing larval forms.

Habitat: Typically found on rocky surfaces in intertidal regions; do not move much over their lifetime.
Morphology:
- Small (2-5 cm), dorsoventrally flattened with a convex dorsal surface.
- 7 to 8 plates on dorsal side, head reduced, mantle cavity extends along the side of the foot.
Feeding: Complete digestive tract; use radula to scrape algae, the presence of magnetite strengthens the radula.
Reproduction: Most have separate sexes; trochophore larva produced.

General characters: Largest, most diverse class; includes snails, limpets, slugs, etc.
Shell characteristics: Always univalve, structures may be coiled/uncoiled; the aperture can be covered by an operculum.
Feeding habits: Use radula for algae scraping, borers, or predation.
Internal forms and functions:
- Respiration by ctenidia (two gills); pulmonates possess a highly vascularized lung.
- Single nephridium for excretion; open circulatory system; well-developed nervous system with various sensory receptors.

General characteristics: Primarily filter feeders, lacking a head and radula.
Body structure and mantle: Shelled in two parts, with ctenidia hanging on either side of visceral mass.
- Mantle covers gills, forming siphons for inhalation/exhalation.
Locomotion: Movement by foot extension, scallops utilize jet propulsion.
Feeding: Mucus secreted to entrap food particles which travel down gills to labial palps directing food to the mouth.
Circulatory system: Open system with a three-chambered heart; U-shaped kidneys for excretion.
Reproduction: Sexually dimorphic with most having external fertilization; glochidia larvae in freshwater forms may live parasitically on fish gills.

General characters: Includes squids, octopuses; regarded as active predators.
Locomotion: Efficient swimmers, capable of rapid backward movement.
Respiration: Uses gills devoid of cilia; water moved by mantle cavity changes in shape.
Circulation: Closed circulatory system with systemic and branchial hearts.
Nervous system: Advanced organ structure; largest brain among invertebrates with a remarkable sense of vision.
Communication: Through chromatophores allowing rapid change in appearance.
Reproduction: Separate sexes, specialized structures for mating, direct development.

Process: Anus, shell, and mantle cavity move from posterior to anterior during development, potentially leading to fouling.
Consequence: Reduction in limb growth; possible evolutionary benefit for internal protection during retraction into the mantle cavity.

Evolution of shell structure: Originating from planispiral designs; allows for more compact arrangements as they grow, promoting a structural adaptation for habitation zones.