Protostomes and Mollusca Overview

Overview of Protostomes

• Protostomes Classification:
  • All protostomes belong to either the Lophotrochozoa or the Ecdysozoa.

Major Clades of Protostomes

• Lophotrochozoa:

  • Embryos develop using spiral cleavage.
  • Mostly aquatic; locomotion through cilia or body contractions.
  • Characterized by:
  • Trochophore: Free-living larval form.
  • Lophophore: Horseshoe-shaped crown of ciliated tentacles for feeding.

• Ecdysozoa:

  • Animals that undergo molting of their exoskeleton.
  • Major groups include Arthropoda and Nematodes.

Phylum Mollusca

• General Characteristics:

  • Second in diversity only to arthropods.
  • Includes snails, slugs, clams, octopuses, etc.
  • Some have shells, others do not.

• Features:

  • Range from microscopic to giant clams (up to 270 kg).
  • Important for human food and economically significant products like pearls and mother-of-pearl.

Mollusks Body Plan

• Mantle:

  • Thick epidermal sheet, bounds mantle cavity, secretes shell (if present).

• Foot:

  • Main locomotion organ, modified into arms or tentacles in cephalopods.

• Internal Organs:

  • Coelom is reduced; concentrated digestive, excretory, and reproductive organs in a visceral mass.
  • Ctenidia: gills in aquatic mollusks, filtering food for bivalves.

• Radula:

  • Rasping structure used for feeding; absent in bivalves.

Circulatory and Excretory Systems

• Circulatory System:

  • Generally open, except for cephalopods which have a closed system.
  • Hemolymph moves through hemocoel, typical hearts are three-chambered.

• Excretory System:

  • Use of nephridia for nitrogenous waste removal.

Reproductive Strategies

• Mostly gonochoric; some are hermaphroditic.
• External fertilization is common, with spiral cleavage in zygote which leads to the trochophore stage.

Classes of Mollusks

• Polyplacophora: Chitons with oval bodies and calcareous plates.
• Gastropoda: Includes limpets, snails, slugs; coiling and torsion in development.
• Bivalvia: Clams, oysters; have no radula, hinged shells, filter-feeders.
• Cephalopoda: Includes squids, octopuses; known for their intelligence and complex behaviors.

Phylum Annelida

• Characteristics:

  • Segmented body plan allows specialization, with repeated units facilitating movement.

• Body Plan:

  • Well-developed head, internal septa dividing segments, each segment has its own organs.
  • Closed circulatory system and a ventral nerve cord facilitate efficient functioning.

Clades of Annelids

• Errantia: Mainly mobile, includes clamworms and sea mice.
• Sedentaria: Many are burrowers or tube dwellers.

Notable Annelids

• Earthworms: Important for soil health; hermaphroditic, cross-fertilize.
• Leeches: Mostly freshwater, some are blood-sucking parasites.

Phylum Arthropoda

• Diversity and Success:
  • Over 1.2 million species; divided into four main groups: Chelicerata, Crustacea, Hexapoda, and Myriapoda.

major groups and characteristics:

• Chelicerata: Scorpions, spiders with specialized chelicerae; body divided into cephalothorax and abdomen.
• Crustacea: Primarily marine, have two pairs of antennae; includes crabs, lobsters.
• Hexapoda: Largest taxonomic group, includes insects; body divided into head, thorax, and abdomen.
• Myriapoda: Centipedes and millipedes; characterized by numerous segments with varying appendages.

Arthropod Traits

• Segmented Body: Allows for specialization and adaptation.
• Exoskeleton: Aids in moisture retention; must undergo molting (ecdysis).
• Jointed Appendages: Provide versatile movement.

Circulatory and Respiration

• Open circulatory systems; respiratory through gills or tracheae in terrestrial species.

Excretory System

• Differentiated systems for aquatic and terrestrial arthropods; Malpighian tubules in insects concentrate waste.

Insectia

• Metamorphosis: Includes both simple (e.g., grasshoppers) and complete (e.g., butterflies) transformations.
• Diverse Orders: Includes beetles, flies, and hymenoptera; known for diverse feeding habits and ecological roles.