Phylum Mollusca: Characteristics and Classification

Phylum Mollusca

• Soft-bodied, mostly protected by a hard shell
• Characterized by muscular foot, visceral mass, and mantle that secretes the shell
• Protostomes, with a water-filled cavity and radula (rasping tongue)
• Torsion, a unique body structure

Classifications of Mollusca

• Gastropoda (snails and slugs)
  • Stomach foot
  • Have an opening, head, and nervous system
  • Simple brain, complete digestive system, and circulation
  • Gas exchange through gills or simple lungs
  • Hermaphroditic, with nephridia present
• Bivalvia (clams and oysters)
  • Often marine, no head, and sessile filter feeders
  • Digestive system complete, gills trap food
  • Rudimentary nervous system, circulation pumped by two-chambered heart
  • Gas exchange through gills
• Cephalopoda (squids, octopus, cuttlefish, and chambered nautiluses)
  • Head-foot, shell is internalized
  • Specialized foot as a tentacle, highly cephalized
  • Highly developed brain and nervous system
  • Closed circulatory system, gill gas exchange

Phylum Arthropoda

• Protostomes, largest phylum
• Important for pollination (economy and environmental)
• Three body parts in insects (head, thorax, abdomen), two in spiders (cephalothorax and abdomen)
• Jointed appendages from thorax, specialized for various functions
• Chitin external skeleton, spiracles, metameric segmentation
• Open blood system, sexes usually separate (females sting)
• Highly motile, developed social behavior

Subphyla of Arthropoda

• Chelicerates
  • Jaw moves side to side
• Mandibulates
  • Have an up and down jaw

Classes of Arthropoda

• Arachnida (spiders, scorpions, and ticks)
  • Cephalothorax and abdomen
  • Four pairs of walking legs, chelicera, pedipalps
  • Well-developed nervous system with sensory hairs and eyes
  • Book lung, open circulation, complete digestive system (bite/inject prey)
  • Malpighian tubule excretion
• Insecta (butterflies, bees, dragonflies)
  • 30 orders of insects, first animals to fly
  • Live in almost every terrestrial habitat, rare in marine habitats
  • Segmented into head, thorax, and abdomen
  • Well-developed brain
• Myriapoda (centipedes and millipedes)
  • Terrestrial, jaw like mandibles
  • Centipedes are carnivores, one pair of leg per segment
  • Millipedes feed on dead organic matter, two pairs of legs per segment
• Crustacea (crabs, shrimp, lobster, crayfish)
  • Aquatic, mandible jaw
  • Branched appendages for feeding and locomotion
  • Many have gill appendages, open circulation
  • Green gland excretion, calcium chitin skeletal system
  • Complex nervous system

Life Cycle of Insects

• Incomplete metamorphism: adult resembles young post-metamorphism (e.g., cockroaches)
• Complete metamorphism: adult does not resemble young at all post-metamorphism (e.g., butterflies)

Social Behavior in Insects

• Honeybees maintain nest temperature at 30°C, even in extreme climates
• They cool the nest by spreading water and fanning their wings
• They also vibrate their wings and bodies to heat up the nest
• Communication in honeybees involves chemical release of pheromones to relay vital information
• Two methods of communication: Round Dance and Waggle Dance

Phylum Echinodermata

• Deuterostomes, with second opening as the mouth
• Coelomates, with tube feet (suction cup)
• Pentaradial symmetry, no head, short complete digestive system
• Can push digestive system outside of their body
• Simple circulation, endoskeleton